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TUCoPS :: Phreaking Cellular - Misc. :: mobfone.txt

IMTS Mobile Phreaking

How to Get into the AT&T Network
by Building Your own Mobile Phone.


     This article is presented for entertainment and academic study only. It is
a violation of Federal laws to operate an unlicensed transmitter or make
fraudulent telephone calls. It is not intended nor expected that anyone
actually build the devices described. The article is simply a detailed and
factual description of something that could be done.

     I wrote a file in collaboration with another telephone experimenter of
high repute on IMTS (Improved Mobile Telephone Service) posted elsewhere on
this board under the title of "Feature Article". This file was downloaded and
posted on another BBS in the Midwest. From there it fell into the hands of the
Chief of Security of Southwestern Bell. His words to the Sysop, who had been
busted for Blue Boxing were, "A person with a knowledge of electronics could
use the information in that file to build his own mobile telephone".
     I am going to explain in this article how you can build your own mobile
phone. If you haven't figured it out already, you will soon see why the
security man was concerned.
     This article presupposes that you have a working knowledge of two-way
radio. If you don't possess this knowledge, get a copy of "The Radio Amateur's
Handbook" (readily available from libraries and book stores) and study up on
narrow band FM and 2-Meter transmitters. To get everything you will need in one
file, I am reprinting the IMTS article here:

Signaling Used in IMTS
(Improved Mobile Telephone Service)

     Each mobile telephone channel consists of two frequencies; one for the
land base station and one for the mobile phone. The base station uses two tones
for signaling:

Idle   2000 Hz
Seize  1800 Hz

The mobiles use three tones:

Guard       2150 Hz
Connect     1633 Hz
Disconnect  1336 Hz

     The land base station marks the idle channel by placing the idle tone on
it. All the mobiles search for the channel with the 2000 Hz idle tone and lock
on to it.
     Each mobile phone is assigned a standard telephone number consisting of
area code + 7 digits. When a land customer dials a mobile number, the idle tone
(2000 Hz) changes to seize (1800 Hz). The number pulsed to the mobile phone
contains 7 digits consisting of the area code and last 4 digits of the number.
The digits are made up of 50 ms pulses of 2000 Hz separated by 50 ms of 1800
     If there is a mismatch between the digits sent and the wired ID in the
mobile, the mobile drops off and hunts for the idle channel. If the number
matches, the mobile will send back an acknowledgement tone of 750 ms of guard
(2150 Hz). The base station waits 3 to 4 seconds for this tone. If not received
in that time, the calling party gets a recording. If the tone is received, the
mobile phone will ring for up to 45 seconds. Ringing is composed of 1800 Hz and
2000 Hz shifting at 25 ms for two seconds then four seconds of 1800 Hz. When
the mobile phone is picked up it sends a connect tone of 1633 Hz for 400 ms to
tell the base station it has answered. When the mobile hangs up, it sends
disconnect, which is 750 ms of 1336 Hz. When the base receives the disconnect
tone, it will drop carrier for about 300 ms and go off. If it is the only
available channel, it will return to idle.
     Now I will describe what happens when a call is originated by a mobile.
When the mobile goes off hook, it sends 350 ms of guard (2150 Hz) followed by
50 ms of connect (1633 Hz). When the base station hears the connect tone, it
removes the idle tone and stays quiet for about 250 ms. It then transmits 250
ms of seize (1800 Hz). The mobile then sends 190 ms of guard and starts
transmitting the ID sequence at 20 pulses per second. The ID is the area code
and last four digits of the mobile's number. The pulses are marked by 25 ms of
connect (1633 Hz) followed by 25 ms of either silence or guard tone (2150 Hz).
If the pulse is odd, it is followed by silence. If even, it is followed by
guard tone. This is used for parity checking. The interdigit time is 190 ms and
will be either silence or guard tone depending on whether the last pulse was
odd or even. If the last pulse of the last digit in the ID is even it will be
followed by 190 ms of guard tone.
     When a number is dialed from a mobile phone, 2150 Hz is sent continuously
as soon a the dial goes off normal (when the dial is moved from its resting
position). Dial pulses representing breaks are marked by 1633 Hz and are sent
at 10 pulses per second. A pulse is 60 ms of 1633 Hz with 40 ms of 2150 Hz
between pulses.
     The most popular mobile telephone channels are located in the VHF high
band. More cities are equipped with these channels than any other band. They
are listed below.

Mobile Telephone Frequencies

Channel     Base     Mobile
-------     ----     ------
  JL       152.51    157.77
  YL       152.54    157.80
  JP       152.57    157.83
  YP       152.60    157.86
  YJ       152.63    157.89
  YK       152.66    157.92
  JS       152.69    157.95
  YS       152.72    157.98
  YR       152.75    158.01
  JK       152.78    158.04
  JR       152.81    158.07


     This is a list of the components you will need to build your own mobile

1. Cassette Tape Recorder.
2. Radio Scanner (Like those used to receive police calls).
3. Mobile phone dialer (build your own).
4. Low Power Transmitter (Modified 2-Meter transmitter 1 - 5 watts).

How to Build a Mobile Phone Dialer

     Build a Wien-Bridge oscillator. These are commonly used in red boxes. If
you don't have a red box schematic, look up Wien-Bridge in an electronics
textbook. Where you would normally connect a frequency adjustment pot, use two
multi-turn pots connected in series. Power for the oscillator will be supplied
by a 9 volt battery.
     Obtain a rotary dial of the type used on rotary telephones. The dial will
have four wires coming out of it; two white, one blue, and one green. The two
white wires make a connection when the dial is off normal (moved from its
resting position). Connect the two white wires in series with one of the leads
from the 9 volt battery. The oscillator will be running only when the dial is
moved off normal. It works like this: Dial is moved off normal. Circuit is
completed between oscillator and battery. Dial goes back to resting position.
Circuit is opened.
     The blue and green wires go to a normally closed contact in the dial. This
contact opens once for each pulse in a dialed digit. For example it opens three
times for the digit "3". Connect these two wires (blue & green) across one of
the pots in the oscillator. With the dial in its resting position, adjust the
other pot for a frequency of 2150 Hz (Guard tone). Move the dial until the
contact opens and adjust the pot with the blue and green wires going to it for
a frequency of 1633 Hz (Connect tone).
     When the dial is moved off normal, power will be applied to the
oscillator, and it will begin running at 2150 Hz. When the dial is released the
short across the second pot will be removed each time the contacts open for a
dial pulse. During these pulse times the frequency will shift down to 1633 Hz.
When the dial gets back to its resting position, power will be removed from the
oscillator. This will exactly duplicate the dial pulsing of a mobile telephone.

The Transmitter

     Antennae used by mobile phone base stations are located on high towers.
This allows line-of-sight transmission to and from the mobiles. If you are
within a few miles of a base station very little power is needed to establish
contact. 1 to 5 watts should be completely adequate. The less power you use,
the less your chances of getting caught. More on this later.
     2-Meter transmitters, used in amateur radio, operate in the range of 144
to 148 Mhz. With a change of crystals and a little retuning, you have your

How to use Your Home brew Mobile Telephone

     With your scanner, locate the base station frequency which currently has
the idle tone on it. Switch to the mobile frequency on that same channel and
monitor it with the cassette recorder running continuously. What you want is a
clean recording of a mobile unit broadcasting its ID sequence. You also want a
recording of the disconnect tone when he hangs up. Once you have these, rewind
the tape to the start of the sequence. Now you are ready to make a call.

The procedure For Placing a Call

1. Set your scanner to the base station frequency with the idle tone and leave
it there. Monitor with earphones to avoid audio feedback through the

2. Set the transmitter to the corresponding mobile frequency. Turn it on and
leave it on.

3. Play the taped ID sequence.

4. Use your dial pulser to call the desired number. If all has gone well, you
will hear your dial pulses in the earphones. You can use this method to call
one of the special 800 numbers and whistle off with 2600 Hz; then MF to
anywhere in the world. This technique will reduce your visibility on the bill
for the ID you are using.

5. When you are ready to hang up, play the disconnect tone and switch off the

A Few Notes About Your Own Security

     You should use only as much transmitter power as necessary to maintain a
reliable contact. If you do much of this kind of experimenting, the FCC is
going to be after you with direction finding equipment. These use directional
antennae and a process of triangulation to locate illegal transmitters. If you
keep your power down, stay mobile, and avoid establishing a pattern of calling
at the same time every day, it will be nearly impossible to track you down.

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