Another choice would be the Ericsson T10, T18, GA628 or A1018s - seriously old phones which cost almost nothing to buy at flea markets, and which have a handy (logic level) serial port. If the battery is u/s, then run the phone directly from a stabilised 5V source, or 4x NiMh AA cells.I would suggest an old Nokia, like the 3310. It uses F-BUS protocol to communicate with, which specifications are available widely on the internet.