Hello everyone 
I am asking the same question, as Eduardo3rd. Code posted by Eduardo3rd to me looks good (when MEGA is used, RXPIN is 10) but still there is a problem! Probably there is a place, where this setting is overrided, but where? It is much more elegant to make code change than bending pin.
Hope someone can help
Eduardo3rd:
Can anyone clarify what RXPIN should be changed to for the ATmega 2560? The following code from the latest version of GSM3SoftSerial.cpp (included with 1.0.5-r2) seems to suggest that it is re-defined depending on the type of AT is connected, but all of my GSM sketches fail when I run them on the 2560 without changes.#if defined(__AVR_ATmega328P__)
#define TXPIN 3
#define RXPIN 2
#define RXINT 3
#elif defined(AVR_ATmega2560) || defined(AVR_ATmega1280)
#define TXPIN 3
#define RXPIN 10
#define RXINT 4
#elif defined(AVR_ATmega32U4)
#define TXPIN 3
#define RXPIN 8
#define RXINT 3
#endif
Do you also have to run a jumper wire somewhere after implementing the change?