Devil,
I had a similar problem. I was using the IComSat PCB with an Arduino Mega. I wrote a simple sketch that sends data received from the SIM900, by the arduino, to my terminal program through my usb port. It also sends data received from the terminal program, by the arduino, to the SIM900. When I first tested this I had the same problem as you. It turns out that I was trying to use the same rx/tx pins for both. Once I realized this I switch to using two sets of rx/tx pins and all was well. Hope this helps.