Hardware
This page lists various modules which could be used in a Homebrew Mobile Phone. NOTE: Some of these items may be hard to source in small quantities. See Suppliers for a list of goodies you can get hold of.
Cellular Wireless Modules
There are a _lot_ of different standards for cellular networks out there. They generally fall under 4 different "Generation". Wikipedia has some useful information about this: GSM Use the selector on the right-hand-side to view information about a particular generation.
Generally, each generation has a different set of frequency bands which they use, however these bands are not universal across the world with some conuntries usings one band and a different country using another. E.g. While a GSM/GPRS phone is always a GSM/GPRS phone, there are 4 different bands which are used across the world.
2.5G (GSM & GPRS): These days 2.5-generation modules are the entry point. They have GSM for voice & SMS and GPRS for data. There are different classes of device which dictate how the GSM & GPRS functionality inter-operate. For more information, try Wikipedia's GPRS classes.
Telit GE863 - A Class 10, Quad-band GSM/GPRS Module. Also features an integrated GPS module. Availability: Main_Page#GSM_Modules
3G:
3.5G:
MC8775V - www.sierrawireless.com