It's going to be FAR more complicated then, but your first job is to download the GSM datasheet and study it.
I would also suggest you should first connect PC's to both GSM modems, and initially write the software on the PC's (due to FAR better and faster programming, testing, and debugging). Once you have the modems doing exactly what you want, then see about porting to the processors you want to use - the PC experience could also determine what type of processor you might need?.