I think you would be better taking your existing hardware, which must be somewhat custom to begin with (since it works over wifi) and use something like a HopeRF module (http://www.hoperf.com/) or similar. HopeRF has transceivers with power output up 100mW, data rates up to 256kbps, and a choice of 4 different bands (315, 433, 868, 915). This sounds like it would be a far easier choice to incorporate into your already-working system than hacking OsmocomBB-capable phones.