Grumpy_Mike:
Any you like apart from 0 & 1 which are used for the hardware serial port.
Are you familiar with the GSM library athttps://github.com/MarcoMartines/GSM-GPRS-GPS-Shield/blob/GSMSHIELD/doc/List.txt
When I decomment the
#define GSM_TXPIN 2
#define GSM_RXPIN 3
lines in GSM.cpp and replace them with my own numbers the Serial input changes. It goes from constantly outputting ATT: OK RIC: to a state of far less output beginning with DB: NO RESP. Both then generally go to setting the baud rate then say ERROR: SIM900 doesn't answer. Check power and serial pins. The GSM shield doesn't work properly in either case.