The Asterisk for Raspberry Pi project is continuously improving with new features and enhancements. The latest feature is particularly interesting, it allows direct calling on GSM/3G networks with USB modems from Huawei and the chan_dongle channel driver. A highly affordable GSM VoIP gateway can be built, using the USB modem as trunk in Asterisk.
You can find complete setup instructions in the documentation section.
A number of interesting applications are possible with this: Least cost routing of course (use the GSM network when it’s cheaper). In remote areas with weak cell phone reception, install the RPi in a location where reception is possible and connect the SIP phones in your house with the RPi. High-gain directional antennas can be connected to some of the USB modems and pointed towards the next cell phone tower, in order to receive weak signals that can’t be received by regular phones anymore. Finally the RPi also makes a good extension to an already existing PBX, enhancing it with GSM gateway capability. With a total power consumption of around 5W for both RPi and the modem, your electricity bill is not streched at all, even if you are only occasionally calling over GSM.
P.S.: If you are interested in this application, you may want to read about this RPi GSM gateway implementation.