Hello folks.

I have this GSM module called SIM900D. It has several interesting capabilites. 1. It has builtin commands AT+SIMEI (guess what it does) and AT*CELLLOCK (ARFCN lock). 2. It, as far as I know, does not check firmware signature. 3. It has an API (called "Embedded AT") which can be called from the customer's module running on the same CPU (ARM926EJS). 4. Non-stripped firmware ELFs are available, together with the descriptions of public API.

Here is a (Russian) page with all of the relevant datasheets collected. Datasheets are in English. http://www.mt-system.ru/catalog/dokumentacija-na-gsm-modemy-simcom

See also: http://www.geekonfire.com/wiki/index.php?title=GPRS_Shield%EF%BC%88SIM900%EF... ftp://ftp.macrogroup.ru/Support/SimCom/Firmware/Sim900/Sim900D