I built one, way back (I'm not an Android dev, kind of hacked it together but on the positive side it's extremely simple: https://github.com/niryariv/KalSMS )

At the time there were a lot of developing countries projects using SMS as their main interface (a lot of work with community health workers etc). Orgs like the UN would set up an SMS gateway by hooking a desktop with some hardware, and of course it would be very vulnerable to stuff like power outages, someone tripping over the wires etc and you had to get the technician to set it up again. With smartphones you had the all the required setup in one cheap, small and very durable package, no training required to operate.