SIM900 schematic verification
SIM900 Serial interface runs on 2.8 V logic levels, PIC runs on 5 V so you are definitely need level shifting. You did not provide a link to that break-out board but I am sure that there must be some level conversion circuit on that.

From application note.


  Please note that the UART level is 2.8V, if the level is not matching, a level shift circuit is needed.