Sorry, explanation must be a little short.
My system is based on an field equipment with a Q24Plus Modem connected to a Front End. The problem found is that, after a perfect boot and exchange of GPRS data, a few of the modems ( 3 out of 200 aprox. ) do not reply to any question of the Front End. Even a response to “ping” is not received. A Customer Application developed with Open AT is running always in the modem. In this case, when a Modem fails to communicate, program is running perfectly and any at comand question by the serial port stands that everything about net registration, gprs registration, network attachment is working well.