DMS - Connecting a modem to a cellphone
Connecting a modem to a cell fone
Phantasm and DaveX

Acoustic Coupling by Phantasm

Since the days of hacking using a standard telephone line are almost over, many of todays hacker community are moving towards cellular communications.

ESN/MIN pairs are no longer difficult to obtain and if used carefully will last much longer than calling cards, codes and most other methods of phone phreaking.

This text file will explain how to easily connect any cellular phone to a modem, without the expense of buying a cellular line interface or similar device.

What you will require:
To undertake this project you will require the following:

  • 1 desktop telephone
  • 1 telephone doubler socket
  • 4 lengths of wire (30cm long)
  • 1 roll of solder
  • 1 roll of masking tape

    You will also require a soldering iron, wire strippers and a screw driver.

    1. Unscrew the screws holding your desktop telephone handset together. Remove the ear piece speaker and microphone from the plastic moulding and cut the wires connected to them.

    2. Now solder the 30cm long lengths of wire onto the existing handset ear piece speaker and microphone wires. Once you have done this, simply re-solder the ear piece speaker and microphone onto the extended handset wiring.

    3. Place the extended handset ear piece speaker over the mouth piece on your cellular telephone and wrap in masking tape. This will not only hold the extended handset ear piece speaker in position, but also prevent background noise problems.

    4. Now place the extended handset microphone over the ear piece on your cellular telephone and wrap in masking tape as above.

    How to use the device:
    Basically, what you have done is turned your desktop telephone into an accoustic coupler type device. To use the device, simply follow the instructions below.

    1. Plug your telephone doubler socket into your telephone socket. Plug your desktop telephone into one socket and your modem line into the other socket.

    2. Leave your telephone handset off the hook until your line is completely dead. (i.e. after you have listened to "please replace the handset and try again" or something similar about 10 times.

    3. Dial the modem dialup you wish to call on your cellular phone and press SEND as you would normally.

    4. Type ATD into your terminal package and press return. Your modem should now be off the hook and waiting for a carrier signal from your cellular telephone.

    Diagram of device:

                                                    :  :
                                                    :  :
           ______                          :             :
          /      :                       /::    :::::    :
         /       :---+   +--------------: ::   :::::::   : ear piece
        /        :-+ :   : +------------: ::    :::::    :
       :     ____: : :   : :             \::  _________  :
       :    /      : :   : :               : :         : :
       :   :       : :   : :               : :_________: :
       :   :       : :   : :               :             :
       :   :       : +-- : : ---------+    :  [1][2][3]  :
       :   :       +---- : : -------+ :    :  [4][5][6]  :
       :   :             : :        : :    :  [7][8][9]  :
       :   :             : :        : :    :  [*][0][#]  :
       :   :             : :        : :    :   _  _  _   :
       :    \____        : :        : :    :  [_][_][_]  :
       :         :       : :        : :  /::             :
        \        :-------+ :        : +-: :: :::         : microphone
         \       :---------+        +---: :: :::         :
          \______:                       \::             :
             :                             :_____________:
             :                            cellular telephone
      desktop telephone
    How it works:
    The carrier signal from the remote modem will be sent out of your cellular telephone ear piece speaker and into the desktop telephone handset microphone. Your modem will receive any data send into the desktop telephone handset after the ATD (off-hook) command has been issued.

    In the same way, any carrier signals from your modem will be sent out of the desktop telephone handset earpiece speaker and into your cellular telephone microphone.

    At the time of writing, I have not connected at speeds higher than 1200 baud expect that those using transportable cellular phones (such as the Motorola 4800 and 6800 series) should have no problems connecting at 2400 baud or higher.

    I did not write this text file so that people could use it to transfer large amounts of data at high speed. This is simply a guide to explain how easy it is to use any cellular telephone with a modem. 1200 baud may be slow, but it is fine for hacking, calling your favourite underground BBS or transfering text files --- especially if you are not paying for it!

    Wiring a modem to a Motorola 4800 by DaveX

                            T1       |     |      T2
          pin 1  o--||--o___   ______o     o______   ___o--||--o pin 6
                            ||                   ||
                            || 1b             2b ||
                        1a  | _____  ____________ | 2a
                            ||     \/            ||
                            || 1c  /\         2c ||
          pin 24 o__________||___/   \__/\/\/\___||_________o pin 24
                                 adjusted to give best
                                 tx/rx isolation
    To connect a modem to a motorola 4800
    You need :-
  • 2 off 2.2uF capacitors. (non polarized)
  • 2 off isolation transformers (signal) with 3 seperate windings
  • 1 off 1k ohm preset
  • 1 off BT socket (to plug modem into)

    Using a capacitor to provide DC isolation, connect winding (a) of transformer 1 to pin 24 on D plug and the other end of winding (a) to pin 1 of the D plug via a capacitor. Connect winding (a) of transformer 2 in a similar manner to pins 24 & 6. (see above).

    Winding 1(b) & 2(b) are connected as in above picture, the start of each winding goes to the BT socket (pins 2 & 5), and the two ends connect to each other.

    Winding 1(c) & 2(c) are connected as in picture, the start of winding 1(c) is connected to the end of winding 2(c), and 2(c) start is connected via a 1k preset to the end of 1(c).

    The way it werks is thus.

    A signal being presented to winding 1(a) will appear on both 1(b) and 1(c). it will then couple to transformer 2 through 2(b) and 2(c), BUT because winding (c) is connected out of phase, it will cancel out the signal induced by winding (b) thus giving no signal at 2(a).

    A signal from 2(a) will follow the same proceedure and not appear at 1(a), but a signal at either 1(a) or 2(a) will be present at the modem. (actually only 50% of the signal will reach the modem, the other 50% is eaten up by the balancing resistor.

    The signal from the modem is delivered to both 1(a) & 2(a), (50% of signal is present at both ports), but as pin 1 is the microphone input to the fone it will accept the signal and send it on its way. Pin 6 on the other hand is the received audio, and it don't care if there is a signal connected to it or not as it is an output.

    The pin numbers given are for the Motorola 45/48/6800 mobile/transportable, but the basic idea should werk on any cellfone. It is also a good idea to disable the microphone and earpiece in the handset once the modem is connected, otherwise (a) the noise from the earpiece gets on your tits, and (b) the microphone tends to pick up your 100 gigawatt stereo system and send the output of same to the remote modem, this causes transmission errors as remote modems don't appreciate or understand 'techno techno techno', at high volume levels.

    Modified version for better cps

                             T1       |     |      T2
    pin 1  o--||-,    ,--o___   ______o     o______   ___o-,   ,---||--o pin 6
                 |    /      ||                   ||     \   |
                 |    \      || 1b             2b ||     /   |
                 |___ /      ||___________________||     \___|
           2.2K preset\  1a  | _____  ____________ | 2a  / 2.2K preset
                      /      ||     \/            ||     \
                      \      || 1c  /\         2c ||     /
          pin 24 o____|______||___/   \__/\/\/\___||_____|____o pin 24
                                 adjusted to give best
                                    tx/rx isolation
    Set all presets to mid position, LOCK MODEM TO 12000 BAUD or 9600 if you not got 12,000 and connect to a USR modem somewhere. Keep adjusting both recieve and transmit presets untill max cps is achieved. (boring, time consuming, fiddly, a real pain in the arse. but worth it.)

    Make sure you set fone volume to max before making adjustments or next time you try and use it, the settings will be wrong.