Julian Macassey, N6ARE

                         First Published
                       Ham Radio Magazine
                         September 1985

            Everybody has one, but what makes it work?
     Although telephones and telephone company practices may vary 
dramatically  from one locality to another, the basic  principles 
underlying the way they work remain unchanged.

     Every  telephone consists of three  separate  subassemblies, 
each capable of independent operation.  These assemblies are  the 
speech  network, the dialing mechanism, and the ringer  or  bell.  
Together, these parts - as well as any additional devices such as 
modems,  dialers,  and answering machines - are attached  to  the 
phone line.

The phone line

     A  telephone is usually connected to the telephone  exchange 
by  about three miles (4.83 km) of a twisted pair of No.22  (AWG) 
or  0.5  mm  copper wires, known by your phone  company  as  "the 
loop".   Although  copper  is  a good  conductor,  it  does  have 
resistance.   The resistance of No.22 AWG wire is 16.46 Ohms  per 
thousand  feet  at 77 degrees F (25 degrees C).   In  the  United 
States,  wire resistance is measured in Ohms per  thousand  feet; 
telephone  companies describe loop length in kilofeet  (thousands 
of  feet).   In  other parts of the  world,  wire  resistance  is 
usually expressed as Ohms per kilometer.
     Because  telephone apparatus is generally considered  to  be 
current   driven,  all  phone  measurements  refer   to   current 
consumption, not voltage.  The length of the wire connecting  the 
subscriber to the telephone exchange affects the total amount  of 
current   that  can  be  drawn  by  anything  attached   at   the 
subscriber's end of the line.

     In  the  United States, the voltage applied to the  line  to 
drive  the telephone is 48 VDC; some countries use 50 VDC.   Note 
that telephones are peculiar in that the signal line is also  the 
power  supply line.  The voltage is supplied by lead acid  cells, 
thus  assuring a hum-free supply and complete  independence  from 
the electric company, which may be especially useful during power 

     At  the telephone exchange the DC voltage and  audio  signal 
are  separated  by  directing  the  audio  signal  through  2  uF 
capacitors and blocking the audio from the power supply with a 5-
Henry choke in each line.  Usually these two chokes are the  coil 
windings  of  a  relay  that switches  your  phone  line  at  the 
exchange;  in the United States, this relay is known as  the  "A" 
relay (see fig.1).  The resistance of each of these chokes is 200 

     We can find out how well a phone line is operating by  using 
Ohm's  law  and  an  ammeter. The DC  resistance  of  any  device 
attached  to the phone line is often quoted in telephone  company 
specifications  as  200  Ohms; this will vary  in  practice  from 
between  150 to 1,000 Ohms. You can measure the DC resistance  of 
your  phone  with an Ohmmeter. Note this is  DC  resistance,  not 

     Using  these figures you can estimate the  distance  between 
your telephone and the telephone exchange.  In the United States, 
the telephone company guarantees you no lower current than 20  mA 
-  or  what is known to your phone company as a  "long  loop."  A 
"short loop" will draw 50 to 70 mA, and an average loop, about 35 
mA.  Some countries will consider their maximum loop as low as 12 
mA.  In practice, United States telephones are usually capable of 
working  at  currents  as  low as 14  mA.   Some  exchanges  will 
consider your phone in use and feed dial tone down the line  with 
currents  as  low as 8 mA, even though the telephone may  not  be 
able to operate.

     Although  the telephone company has supplied plenty of  nice 
clean  DC  direct  to your home, don't assume  you  have  a  free 
battery  for your own circuits.  The telephone company wants  the 
DC resistance of your line to be about 10 megOhms when there's no 
apparatus  in use ("on hook," in telephone company  jargon);  you 
can  draw no more than 5 microamperes while the phone is in  that 
state.   When  the phone is in use, or "off hook," you  can  draw 
current, but you will need that current to power your phone,  any 
current you might draw for other purposes would tend to lower the 
signal level.

     The  phone  line has an impedance  composed  of  distributed 
resistance, capacitance, and inductance.  The impedance will vary 
according  to the length of the loop, the type of  insulation  of 
the wire, and whether the wire is aerial cable, buried cable,  or 
bare  parallel wires strung on telephone poles.  For  calculation 
and specification purposes, the impedance is normally assumed  to 
be 600 to 900 Ohms.  If the instrument attached to the phone line 
should  be of the wrong impedance, you would get a  mismatch,  or 
what  telephone  company  personnel refer to  as  "return  loss."  
(Radio  Amateurs will recognize return loss as SWR.)  A  mismatch 
on telephone lines results in echo and whistling, which the phone 
company  calls "singing" and owners of very cheap telephones  may 
have  come  to expect.  A mismatched device can, by the  way,  be 
matched  to  the phone line by placing resistors in  parallel  or 
series  with  the line to bring the impedance of  the  device  to 
within the desired limits.  This will cause some signal loss,  of 
course, but will make the device usable.

     A  phone  line  is balanced feed,  with  each  side  equally 
balanced  to ground.  Any imbalance will introduce hum and  noise 
to the phone line and increase susceptibility to RFI.

     The  balance  of the phone line is known to  your  telephone 
company  as "longitudinal balance."  If both impedance match  and 
balance  to ground are kept in mind, any device attached  to  the 
phone  line  will perform well, just as the correct  matching  of 
transmission  lines and devices will ensure good  performance  in 
radio practice.

     If  you  live  in the United States,  the  two  phone  wires 
connected  to your telephone should be red and green.  (In  other 
parts  of the world they may be different colors.)  The red  wire 
is  negative  and  the green wire is  positive.   Your  telephone 
company  calls the green wire "Tip" and the red wire "Ring".  (In 
other parts of the world, these wires may be called "A" and "B".)  
Most installations have another pair of wires, yellow and  black.  
These wires can be used for many different purposes, if they  are 
used  at all.  Some party lines use the yellow wire as a  ground; 
sometimes  there's  6.8 VAC on this pair to light  the  dials  of 
Princess type phones.  If you have two separate phone lines  (not 
extensions) in your home, you will find the yellow and black pair 
carrying  a second telephone line.  In this case, black is  "Tip" 
and yellow is "Ring."

     The  above description applies to a standard line with a  DC 
connection  between  your  end  of the  line  and  the  telephone 
exchange.  Most phone lines in the world are of this type,  known 
as  a "metallic line."  In a metallic line, there may or may  not 
be  inductance devices placed in the line to alter the  frequency 
response  of  the line; the devices used to do  this  are  called 
"loading  coils."  (Note: if they impair the  operation  of  your 
modem,  your telephone company can remove them.)  Other types  of 
lines  are party lines, which may be metallic lines  but  require 
special   telephones   to   allow  the   telephone   company   to 
differentiate  between  subscribers.  Very long  lines  may  have 
amplifiers,  sometimes  called "loop extenders"  on  them.   Some 
telephone  companies  use a system called  "subscriber  carrier," 
which is basically an RF system in which your telephone signal is 
heterodyned  up  to around 100 Khz and then  sent  along  another 
subscriber's "twisted pair."

     If  you have  questions about your telephone line,  you  can 
call your telephone company; depending on the company and who you 
can reach, you may be able to obtain a wealth of information.

The Speech Network

     The speech network - also known as the "hybrid" or the  "two 
wire/four  wire network" - takes the incoming signal and feeds it 
to the earpiece and takes the microphone output and feeds it down 
the line.  The standard network used all over the world is an  LC 
device  with a carbon microphone; some newer phones use  discrete 
transistors or ICs.

     One  of  the advantages of an LC network is that it  has  no 
semiconductors,   is  not  voltage  sensitive,  and   will   work 
continuously  as  the voltage across the line is  reduced.   Many 
transistorized phones stop working as the voltage approaches 3 to 
4 Volts.

     When  a  telephone is taken off the hook, the  line  voltage 
drops  from 48 Volts to between 9 and 3 Volts, depending  on  the 
length  of the loop.  If another telephone in parallel  is  taken 
off the hook, the current consumption of the line will remain the 
same and the voltage across the terminals of both telephones will 
drop.  Bell Telephone specifications state that three  telephones 
should  work in parallel on a 20 mA loop;  transistorized  phones 
tend  not to pass this test, although some manufacturers use  ICs 
that will pass.  Although some European telephone companies claim 
that phones working in parallel is "technically impossible,"  and 
discourage  attempts  to make them work that way, some  of  their 
telephones will work in parallel.

     While  low levels of audio may be difficult to hear,  overly 
loud  audio  can  be  painful.   Consequently,  a  well  designed 
telephone  will  automatically adjust its  transmit  and  receive 
levels  to allow for the attenuation - or lack of it - caused  by 
the  length  of  the  loop.   This  adjustment  is  called  "loop 
compensation."   In  the United States,  telephone  manufacturers 
achieve  this  compensation with silicon carbide  varistors  that 
consume  any  excess  current from a short  loop  (see  fig.  2).  
Although   some   telephones  using  ICs   have   built-in   loop 
compensation,  many  do  not; the latter have  been  designed  to 
provide  adequate  volume on the average loop, which  means  that 
they provide low volume on long loops, and are too loud on  short 
loops.   Various  countries  have  different  specifications  for 
transmit  and receive levels; some European countries  require  a 
higher transmit level than is standard in the United States so  a 
domestically-manufactured telephone may suffer from low  transmit 
level if used on European lines without modification.

     Because  a telephone is a duplex device,  both  transmitting 
and receiving on the same pair of wires, the speech network  must 
ensure  that not too much of the caller's voice is fed back  into 
his  or  her  receiver.  This  function,  called  "sidetone,"  is 
achieved  by phasing the signal so that some cancellation  occurs 
in  the speech network before the signal is fed to the  receiver.  
Callers  faced  with no sidetone at all will consider  the  phone 
"dead."   Too little sidetone will convince callers that  they're 
not  being heard and cause them to shout, "I can hear  you.   Can 
you  hear ME?"  Too much sidetone causes callers to  lower  their 
voices and not be heard well at the other end of the line.

     A  telephone on a short loop with no loop compensation  will 
appear  to have too much sidetone, and callers will  lower  their 
voices.   In this case, the percentage of sidetone is  the  same, 
but  as the overall level is higher the sidetone level will  also 
be higher. 

The Dial

     There  are two types of dials in use around the world.   The 
most common one is called pulse, loop disconnect, or rotary;  the 
oldest form of dialing, it's been with us since the 1920's.   The 
other  dialing  method,  more  modern and  much  loved  by  Radio 
Amateurs  is called Touch-tone, Dual Tone Multi-Frequency  (DTMF) 
or  Multi-Frequency (MF) in Europe. In the U.S. MF  means  single 
tones used for system control.

     Pulse  dialing is traditionally accomplished with  a  rotary 
dial,  which is a speed governed wheel with a cam that opens  and 
closes a switch in series with your phone and the line.  It works 
by  actually  disconnecting  or "hanging  up"  the  telephone  at 
specific intervals.  The United States standard is one disconnect 
per   digit,   so  if  you   dial  a  "1,"  your   telephone   is 
"disconnected" once.   Dial a seven and you'll be  "disconnected" 
seven times; dial a zero, and you'll "hang up " ten times.   Some 
countries  invert the system so "1" causes ten "disconnects"  and 
0,  one disconnect.  Some add a digit so that dialing a  5  would 
cause six disconnects and 0, eleven disconnects.  There are  even 
some  systems in which dialing 0 results in one  disconnect,  and 
all  other digits are plus one, making a 5 cause six  disconnects 
and 9, ten disconnects.

     Although  most exchanges are quite happy with rates of 6  to 
15  Pulses Per Second (PPS), the phone company accepted  standard 
is  8  to  10 PPS.  Some modern digital exchanges,  free  of  the 
mechanical  inertia problems of older systems, will accept a  PPS 
rate as high as 20. 

     Besides  the PPS rate, the dialing pulses have a  make/break 
ratio,  usually  described as a percentage, but  sometimes  as  a 
straight  ratio.  The North American standard is  60/40  percent; 
most of Europe accepts a standard of 63/37 percent.  This is  the 
pulse measured at the telephone, not at the exchange, where  it's 
somewhat  different, having traveled through the phone line  with 
its  distributed  resistance, capacitance,  and  inductance.   In 
practice,  the  make/break  ratio does not  seem  to  affect  the 
performance of the dial when attached to a normal loop.  Bear  in 
mind that each pulse is a switch connect and disconnect across  a 
complex  impedance, so the switching transient often reaches  300 
Volts.   Try  not  to  have your fingers  across  the  line  when 

     Most pulse dialing phones produced today use a CMOS IC and a 
keyboard.  Instead of pushing your finger round in circles,  then 
removing  your finger and waiting for the dial to  return  before 
dialing the next digit, you punch the button as fast as you want.  
The  IC stores the number and pulses it out at the  correct  rate 
with the correct make/break ratio and the switching is done  with 
a high-voltage switching transistor.  Because the IC has  already 
stored the dialed number in order to pulse it out at the  correct 
rate,  it's a simple matter for telephone designers to  keep  the 
memory  "alive"  and allow the telephone to  store,  recall,  and 
redial the Last Number Dialed (LND).  This feature enables you to 
redial by picking up the handset and pushing just one button.

Because pulse dialing entails rapid connection and  disconnection 
of  the phone line, you can "dial" a telephone that has lost  its 
dial,  by  hitting  the hook-switch rapidly.   It  requires  some 
practice to do this with consistent success, but it can be  done.  
A  more sophisticated approach is to place a Morse key in  series 
with  the  line, wire it as normally closed and send  strings  of 
dots corresponding to the digits you wish to dial.

     Touch  tone,  the most modern form of dialing, is  fast  and 
less  prone to error than pulse dialing.  Compared to pulse,  its 
major  advantage is that its audio band signals can  travel  down 
phone  lines further than pulse, which can travel only as far  as 
your  local  exchange.   Touch-tone can  therefore  send  signals 
around  the  world via the telephone lines, and can  be  used  to 
control phone answering machines and computers.  Pulse dialing is 
to  touch-tone as FSK or AFSK RTTY is to Switched  Carrier  RTTY, 
where mark and space are sent by the presence or absence of DC or 
unmodulated  RF carrier.  Most Radio Amateurs are  familiar  with 
DTMF for controlling repeaters and for accessing remote and  auto 
phone patches.

     Bell  Labs developed DTMF in order to have a dialing  system 
that  could travel across microwave links and work  rapidly  with 
computer  controlled exchanges.  Each transmitted digit  consists 
of two separate audio tones that are mixed together (see  fig.3).  
The  four  vertical columns on the keypad are known as  the  high 
group and the four horizontal rows as the low group; the digit  8 
is  composed  of 1336 Hz and 852 Hz.  The level of each  tone  is 
within  3  dB  of the other, (the telephone  company  calls  this 
"Twist").  A complete touch-tone pad has 16 digits, as opposed to 
ten on a pulse dial.  Besides the numerals 0 to 9, a DTMF  "dial" 
has *, #, A, B, C, and D.  Although the letters are not  normally 
found  on consumer telephones, the IC in the phone is capable  of 
generating them.

     The  * sign is usually called "star" or "asterisk."   The  # 
sign,  often referred to as the "pound sign." is actually  called 
an  octothorpe.  Although many phone users have never used  these 
digits  -  they are not, after all, ordinarily  used  in  dialing 
phone  numbers  -  they  are used  for  control  purposes,  phone 
answering machines, bringing up remote bases, electronic banking, 
and repeater control.  The one use of the octothorpe that may  be 
familiar occurs in dialing international calls from phones in the 
United  States.  After dialing the complete number,  dialing  the 
octothorpe  lets the exchange know you've finished  dialing.   It 
can now begin routing your call; without the octothorpe, it would 
wait and "time out" before switching your call.

     When DTMF dials first came out they had complicated cams and 
switches   for  selecting  the  digits  and  used  a   transistor 
oscillator  with  an  LC tuning network to  generate  the  tones.  
Modern  dials use a matrix switch and a CMOS IC that  synthesizes 
the  tones  from  a  3.57MHz  (TV  color  burst)  crystal.   This 
oscillator  runs  only  during dialing, so  it  doesn't  normally 
produce QRM.

     Standard DTMF dials will produce a tone as long as a key  is 
depressed.   No  matter  how long you press,  the  tone  will  be 
decoded as the appropriate digit.  The shortest duration in which 
a  digit can be sent and decoded is about 100 milliseconds  (ms).  
It's  pretty  difficult  to dial by hand at  such  a  speed,  but 
automatic dialers can do it.  A twelve-digit long distance number 
can  be  dialed by an automatic dialer in a little  more  than  a 
second - about as long as it takes a pulse dial to send a  single 
0 digit.

     The output level of DTMF tones from your telephone should be 
between  0 and -12 dBm.  In telephones, 0 dB is 1  miliwatt  over 
600  Ohms.   So 0 dB is 0.775 Volts.  Because your  telephone  is 
considered  a 600 Ohm load, placing a voltmeter across  the  line 
will enable you to measure the level of your tones. 

The Ringer

     Simply  speaking  this  is a device that alerts  you  to  an 
incoming  call.  It may be a bell, light, or warbling tone.   The 
telephone company sends a ringing signal which is an AC waveform.  
Although the common frequency used in the United States is 20 HZ, 
it can be any frequency between 15 and 68 Hz.  Most of the  world 
uses  frequencies  between  20 and 40 Hz.   The  voltage  at  the 
subscribers  end depends upon loop length and number  of  ringers 
attached to the line; it could be between 40 and 150 Volts.  Note 
that  ringing voltage can be hazardous; when you're working on  a 
phone line, be sure at least one telephone on the line is off the 
hook  (in  use); if any are not, take high  voltage  precautions.  
The  telephone  company may or may not remove the 48  VDC  during 
ringing;  as  far  as you're concerned, this  is  not  important.  
Don't take chances. 

     The  ringing  cadence  - the timing of ringing  to  pause  - 
varies from company to company.  In the United States the cadence 
is  normally  2  seconds of ringing to 4 seconds  of  pause.   An 
unanswered phone in the United States will keep ringing until the 
caller  hangs up.  But in some countries, the ringing will  "time 
out" if the call is not answered.

     The  most  common  ringing  device is  the  gong  ringer,  a 
solenoid  coil  with a clapper that strikes either  a  single  or 
double bell.  A gong ringer is the loudest signaling device  that 
is solely phone-line powered.

     Modern  telephones tend to use warbling ringers,  which  are 
usually  ICs powered by the rectified ringing signal.  The  audio 
transducer  is either a piezoceramic disk or a small  loudspeaker 
via a transformer.

     Ringers  are  isolated from the DC of the phone  line  by  a 
capacitor.   Gong  ringers  in the United States use  a  0.47  uF 
capacitor.  Warbling ringers in the United States generally use a 
1.0  uF  capacitor.  Telephone companies in other  parts  of  the 
world  use  capacitors  between  0.2  and  2.0  uF.   The   paper 
capacitors of the past have been replaced almost exclusively with 
capacitors  made of Mylar film.  Their voltage rating  is  always 
250 Volts.

     The  capacitor  and  ringer coil, or Zeners  in  a  warbling 
ringer,  constitute a resonant circuit.  When your phone is  hung 
up ("on hook") the ringer is across the line; if you have  turned 
off  the  ringer  you have merely silenced  the  transducer,  not 
removed the circuit from the line.

     When the telephone company uses the ringer to test the line, 
it  sends  a  low-voltage, low frequency  signal  down  the  line 
(usually  2 Volts at 10 Hz) to test for continuity.  The  company 
keeps records of the expected signals on your line.  This is  how 
it  can  tell  you have added equipment to your  line.   If  your 
telephone has had its ringer disconnected, the telephone  company 
cannot detect its presence on the line.

     Because there is only a certain amount of current  available 
to  drive ringers, if you keep adding ringers to your phone  line 
you will reach a point at which either all ringers will cease  to 
ring, some will cease to ring, or some ringers will ring  weakly.  
In  the  United States the phone company will guarantee  to  ring 
five  normal ringers.  A normal ringer is defined as  a  standard 
gong  ringer  as  supplied  in  a  phone  company  standard  desk 
telephone.   Value  given to this ringer  is  Ringer  Equivalence 
Number  (REN)  1.  If you look at the FCC registration  label  of 
your  telephone, modem, or other device to be connected   to  the 
phone line, you'll see the REN number.  It can be as high as 3.2, 
which  means  that device consumes the equivalent  power  of  3.2 
standard ringers, or 0.0, which means it consumes no current when 
subjected  to  a  ringing  signal.  If  you  have  problems  with 
ringing,  total  up your RENs; if the total is  greater  than  5, 
disconnect ringers until your REN is at 5 or below.

     Other  countries  have various ways of expressing  REN,  and 
some  systems  will handle no more than three of  their  standard 
ringers.  But whatever the system, if you add extra equipment and 
the  phones  stop ringing, or the phone answering  machine  won't 
pick  up  calls,  the solution is disconnect  ringers  until  the 
problem  is resolved. Warbling ringers tend to draw less  current 
than  gong  ringers, so changing from gong  ringers  to  warbling 
ringers may help you spread the sound better.

     Frequency response is the second criterion by which a ringer 
is  described.   In  the  United States  most  gong  ringers  are 
electromechanically  resonant.  They are usually resonant  at  20 
and  30 Hz (+&- 3 Hz).   The FCC refers to this as A so a  normal 
gong ringer is described as REN 1.0A.  The other common frequency 
response  is  known as type B.  Type B ringers  will  respond  to 
signals between 15.3 and 68.0 Hz.  Warbling ringers are all  type 
B  and some United States gong ringers are type B.   Outside  the 
United States, gong ringers appear to be non-frequency selective, 
or type B.

     Because a ringer is supposed to respond to AC waveforms,  it 
will tend to respond to transients (such as switching transients) 
when the phone is hung up, or when the rotary dial is used on  an 
extension phone.  This is called "bell tap" in the United States; 
in  other  countries,  it's often called  "bell  tinkle."   While 
European  and  Asian phones tend to bell tap, or  tinkle,  United 
States ringers that bell tap are considered defective.  The  bell 
tap  is  designed out of gong ringers and fine  tuned  with  bias 
springs.   Warbling  ringers  for use in the  United  States  are 
designed  not  to respond to short transients;  this  is  usually 
accomplished  by  rectifying the AC and filtering  it  before  it 
powers the IC,  then not switching on the output stage unless the 
voltage lasts long enough to charge a second capacitor.


     This  brief  primer  describing  the  working  parts  of   a 
telephone is intended to provide a better understanding of  phone 
equipment.    Note  that  most  telephone  regulatory   agencies, 
including the FCC, forbid modification of anything that has  been 
previously approved or attached to phone lines.   

                   End of text. Figures Follow

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

                      Fig 1. The Phone Line

                A RELAY
                200 Ohms   Telephone    . Subscriber
                -------    Exchange     .
                -------                 .  TIP +
          |       5 H    |              .
          |              |              .
         +|              |              .
         ---             |              .    No 22 AWG wire
         --- 48V DC      |              .    up to 10 Miles Long
          -              |              .
         ---    A RELAY  |              .
         -|     200 Ohms |              .
          |     -------  |              .
          |     -------  |              . RING -
                  5 H    |         |    .
          Audio      2uF |     2uF |    .
          coupling 250V ---  250V ---
          Capacitors    ---       ---
                         |         |
          o----- \--------         |
               A RELAY Contacts    |
          o----- \------------------    

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

                Fig 2. Telephone Speech Network.

                Simplified U.S. Standard "425B". Component Values 
may  vary between manufacturers. Connections for  Dials,  Ringers 
etc. not shown.

                       . |                  .|    
     Sidetone balancing. |  0.047uF 250V    .|
     impedance & loop  . |    | |           .|
     compensation. >>> . o----| |-------o   .|
                       . |    | |       |   .|
                       . |              |   .|
                       . |    |<| VR2   |   .|
                       . o----| |-------o---.|
                       . |    |>|          |.|
                       . |                 |.|
                       . |   68 Ohms       |.|
                       . o---\/\/\/-----|  |.|
                         |              |  | |
                         |        .     |  | |
                         -----)||(------|---------o (GN)
                             1)||(5     |  | |    |
               Loop           )||(      |  | |    |
     TIP       Compensation  2)||(6     |  | |    |
     o------ \------o---------)||(------o  | | RX O
            .       | (RR)   . ||       |  | |    |
            .       |          || 1.5uF |  | |    |
            .       \ 180      ||      --- | |    |
            .       / Ohms     ||      --- | |----o (R)
            .       \          || 250V  |  |      |
            .       |          ||       |  |      |    
            .  VR1 ---       . || .     |  |      |
            .      ^ ^    ----)||(------o---   TX O
            .      ---    |  3)||(7               |
            .       |     |   )||(                |
      RING  .       | (C) |  4)||(8       22 Ohms |
     o----- \-------o---------)||(---o----/\/\/---o (B) 
                          |          |
            ^             |          |  
        Hookswitch        ------------

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fig. 3.   Standard DTMF pad and Frequencies

   (Low    ____      ____      ____      ____ 
    Group)|    |    |    |    |    |    |    |
   697Hz >|  1 |    |  2 |    |  3 |    |  A |            
          |____|    |____|    |____|    |____|      

           ____      ____      ____      ____ 
          |    |    |    |    |    |    |    |
   770Hz >|  4 |    |  5 |    |  6 |    |  B |
          |____|    |____|    |____|    |____|

           ____      ____      ____      ____ 
          |    |    |    |    |    |    |    |
   825Hz >|  7 |    |  8 |    |  9 |    |  C |
          |____|    |____|    |____|    |____|

           ____      ____      ____      ____ 
          |    |    |    |    |    |    |    |
   941Hz >|  * |    |  0 |    |  # |    |  D |
          |____|    |____|    |____|    |____|

            ^         ^         ^         ^  
          1209Hz    1336Hz    1477Hz    1633Hz
                     (High Group)