Did you try to use a ISDN modem or better a call from one GM29 to other GM29?
By GSM to GSM call you can be nearly 100% to get the bearer data, but may no bearer data if the base station does not send any bearer code. Please make a GSM to GSM call and tell the result.
Regards
Meff
GSM / GPS / GPRS modem and module for vehicle tracking