The Media Lab at MIT has published blueprints for an open source GSM cellular phone. It's supposed to work with any GSM provider. Accepts any SIM card, uses a GSM module from Sparkfun. Sports a TFT display. The repo includes source code for the firmware, Eagle design files for the circuit board, and Inkscape design files for laser cutting the casing.
open-source DIY mit cellphone gsm software hardware repository github project