GSM Modules

Introduction

GSM modules are often used in high altitude ballooning as a backup telemetry system. Due to the horziontal plane in which GSM networks are setup (to maximise reception on the ground) GSM doesn't work above approximately 2km altitude. If the module tries to send a SMS it just times out when the module is above the max altitude. On returning back down to below 2km the module is able to get a connection and can then start sending messages again.
There are a number of methods available to connect GSM modules to your flight computer to act as another form of communication.

ASCII interface
Phone may or may not accept SMSes in plain text - you may need to convert to PDU code. This is relatively easy, and the binary result is hex-dumped as ASCII before being sent to the phone.
REQUIRES \r\n (CRLF)!!! Linux users beware, don't waste your time wondering why it doesn't work!

Binary Serial, No parity, 1 stop bit. Checksums are included as part of the protocol, the last two bytes. Refer to http://www.embedtronics.com/nokia/fbus.html.
The Linux Gnokii project, while not documenting FBUS, implements the FBUS protocol as part of it's program.

MBUS is harder to implement on the serial level, but has a much slower baudrate than FBUS, which may be useful for micro controllers.

The Nokia 5110 was used on: Pegasus I, Pegasus III to great effect.
All Nokias have FBUS and MBUS 'ports' - while the MBUS is a bit strange with RX and TX over one wire, FBUS is a nice, simple serial with a binary protocol.
FBUS communication is done over three wires (GND, RX, TX), whereas MBUS is done over two (GND, RX&TX on one wire).
Exposed contacts for FBUS are usually available on the outside of the phone only for older models. New Nokias have gold pads on the inside which are incredibly difficult to solder as they are gold plated.
Recommended to get a cable rather then soldering to the pins

The w800 accepts AT commands over a 9600 baud serial.
Although a USB cable can be used to connect this phone to the computer and harness the serial, connecting the phone to a microprocessor's UART is ~~harder~~ pretty much impossible, unless you fancy buying or writing a USB host controller.
the T86i has 2.8v 9600 baud serial broken out - This is the easiest solution, as the UART is physically exposed. This pinout is correct, and the pins required are pin 4 and 10. A DKU-11 USB cable works fine if you don't fancy soldering to the phone itself, but it's still difficult to hack. Many payloads have achieved success with this phone!