SIM800L - AT Commands not doing anything on Arduino Serial Monitor
Do not use SoftwareSerial on a Mega2560 unless you absolutely know what you are doing.  

SoftwareSerial does not work on many of the pins of the Mega2560, and since there are other hardware serial ports available there is seldom any reason to resort to using SoftwareSerial anyway.

Connect your SIM800L to the TX1/RX1 pins of the Mega2560 then communicate using the pre-defined Serial1 object.