Upgraded branch of the Adafruit FONA C++ library for handling incoming caller's number retrieve. This feature, not implemented yet in the official Adafruit library, is particularly interesting if you want to perform different actions based on the calling number or simply log the caller's number. My implementation is interrupt-based and so not blocking. According to the SimCom 800L documentation, when a phone call occurs an hardware interrupt is thrown by the module and can be handled by the RI FONA output pin that in my implementation is tied to Arduino interupt 0 (pin 3 in Arduino Uno Rev3).