The processor that interprets those AT commands run its own operating system, and it runs a huge codebase implementing at the very least the GSM layer 2/3 and the UMTS layer 2/3 protocols. (Here's(http://cgit.osmocom.org/vovida-gsml/tree/) partially leaks of such code for an old GSM only phone, ramping up at about near 200000 lines of C and C++ code)
It sounds like you don't really have access to that and can e.g. change the protocol messages being exchanged by the BTS/NodeB or the mobile switching center.
It sounds like you don't really have access to that and can e.g. change the protocol messages being exchanged by the BTS/NodeB or the mobile switching center.