The SIM300 is no longer manufactured by SIMCOM, but instead the upgrade is now the SIM900. The AT commands remain the same, so these commands can stay the same
Their is a paperback book on how to interface a Microchip PIC to the SIM300 written a couple of years ago. The book was called "Microcontroller Based GSM/GPRS Projects: Advanced Microcontroller Projects" by Goan Ibraham & Ahmet Ibrahim and is sold on Amazon which might help with your question.