What a great idea. I'm thinking of combining this with a 3D printer to make a custom housing. Some privacy enthusiasts will love this. Cool project.
It uses GSM, which can be listened to by a RTL-SDR or any other software defined radio.
http://www.rtl-sdr.com/receiving-decoding-decrypting-gsm-sig...
No successful or legal opensource/openhardware solutions exist right now :/ This was one of main problems with OpenMoko opensource phone.