In most parts of the world, POTS is 64 kbit/s, over which you can reliably run a 56 kbit/s modem. Cell phone audio uses less bandwidth, but I can imagine that you could get 8 kbit/s or more reliably, compared to the SMS data rate (7 bits/char * 160 chars/msg * 3 msg/sec = 3.4 kbit/s).