Here is a DIY cellphone based on the Arduino platform and the GSM shield. It features basic functionality of a cellphone including making calls, sending SMS, storing phone numbers and displaying the current time.
The DIY Cellphone is a working (albeit basic) cellphone that you can make yourself. It can make and receive phone calls and text messages, store names and phone numbers, and display the time. It builds on the hardware and software in the Arduino GSM Shield but extends it with a full interface, including display, buttons, speaker, microphone, etc.
Via the Electronics Lab.