Typical! I've not come across that one before and unfortunately with it using the RS232 interface I'm not really able to help any further.
All I can suggest is getting a copy of Serial Relay installed on your Arduino and try swapping Rx/Tx over and trying different baud rates to the SIM900 (SIM900.begin). Give it enough time to get a network lock and then send an AT command and see if it responds. Once you find the magic combination load your own code and take it from there! Simple! 