I don't know if you already solve your problem, but I found one solution :-)
I had the same problem that you described. I had no received any response for AT commands. I lost many time searching for one solution, and after read the datasheet of the SIM900, I found the PINs responsable by the TX and RX, they are PIN 9 and 10, respectively. On the "EFcom Shield" there are one way to access those PINs directly. If you look bellow the Shield, you can view the labels RX and TX (located bellow the PINs responsable by NOKIA5110_LCD), in other words, if you use RX instead of S_RX and use TX instead of S_TX everything will work perfectly.