GSM Modem Transmission Issue
The issue was in fact a clock error as pointed out in the comments in my original question. There was a two clock cycle delay from when I pulled the transmission line from low to high to signal the start and then again from high to low to signal the stop (from one of my other entities that was responsible for the TX_READY signal being toggled). Digging up on the manual I found it uses a clock check to determine when its ready to transmit the data to the phone and the delays caused the modem to reset it's buffer and then transmit whatever it received, which because of the delay was nothing. 

Thank you to users Sourabh Tapas and Chris Stratton for helping me identify these issues.