Similarly to its competitors, truphone offers free mobile calls to other truphone users, but charges for all other calls. Dissimilarly, truphone is almost completely built with open source code. More about their technology follows, “Truphone calls are routed via Wi-Fi using Session Initiation Protocol (SIP) and Real-time Transport Protocol (RTP), as shown below. The RTP proxy deals with the Network Address Translation (NAT) used by firewalls, and with transcoding from the Adaptive Multi-Rate (AMR) codec used in the phone to G.711 if required for transmission to the Public Switched Telephone Network (PSTN) via the SIP router. The handset software is written in C++ on the Symbian operating system.” More technical information about truphone’s calling process can be found here. truphone is currently supported by Nokia E and N series phones with other symbian phones on the way most likely.