Support the author of the question, recently also puzzled searching such software and have not found anything for the Android platform, however I am absolutely sure, and useful only. VoIP uses more people and I'm sure increased the number of "builders" home VoIP systems, which and rank. I have a need for incoming calls land lines to send to cloud PBX purely receive incoming calls from the PSTN FXO all just bought a box from Dlink DVG7111S and happiness seemed and GSM as well, but it was not there, first look for "boxes" type Dlink, was in a similar budget mainly adapters wired phone with GSM FXS channel, theoretically I think the Dlink can connect to a FXS port cons not sure about the result, buying another "box", reminiscent of the decision by train....
Other "boxes" too, multichannel, 4, 8, 16, and the price tag tens of Tr and a are box for connecting a USB GSM modem, I have such a modem, but even in prof the solution from scratch and modems to purchase, in short no.
About GSM modem say have a working solution with the help of the programs from Oktel, saw works great, but only under Windows, so it's either PC to drive 24x7x365, someone will say buy nettop the answer in the word buy.... for one number whole nettop doubtful.
And the thought came that I found this thread with a question. Namely, any old not used for any other purpose it had, I think many is good you already have. In a smart for Android has all the hardware required to implement the functionality of the GSM VoIP gateway. GSM module for receiving (sending) calls from network to network, WiFi to communicate with the "fat" home Internet, the processor is not worse than in different "boxes" which quite overpower this task, and even more battery on Board in case of momentary power, can't find the main app with such functions.