Hi! I would suggest to select the Mobile terminal device of your choice first and study the specs and AT commands that accompany it. Most of the AT commands are common but each terminal might have its own special ones.

Also I would strongly recommend to use the UART module of the PIC's and also use bidirectional communication to check that the device has replied with OK after succesful command.

For example:

send AT+cr+lf

device replies OK+cr+lf

After you establish a good command of the device you are going to use then trie to send SMS. It is a difficult task for the begginer.

Ioannis