Hello. This help me and I think that can help you too.
A possible solution for who is having problem with Arduino Mega and SoftwareSerial, I find on a site my solution:
Digging around in the source code of the library, I came accross the following:
// Specifically for the Arduino Mega 2560 (or 1280 on the original Arduino Mega)
// A majority of the pins are NOT PCINTs, SO BE WARNED (i.e. you cannot use them as receive pins)
// Only pins available for RECEIVE (TRANSMIT can be on any pin):
// (I've deliberately left out pin mapping to the Hardware USARTs - seems senseless to me)
// Pins: 10, 11, 12, 13, 50, 51, 52, 53, 62, 63, 64, 65, 66, 67, 68, 69
So pins '2 & 3 won't work (for receive). Maybe this post can help someone else that has the same problem.'''