I'm working with an Arduino Mega ADK and Arduino's GSM Shield. I followed the instructions for jumpering the pin 2 across to pin 10 for interrupts on the Mega. However, I cannot get Arduino's "TestModem" sketch to identify a modem. The instruction says no SIM card required, is this wrong? I have the On LED lit, the Status LED lit, and the Net LED is flashing. Nothing else is plugged into any port on the Arduino right now except the GSM shield. Power is being supplied from the USB on my laptop. Thank you!