Answering a phone call with RN-52 and Arduino Mega 2560
Serial.begin(9600); //start talking to RN52



I think you need to pull GPIO7 down to set speed to 9600.

From datasheet


  GPIO7 Driving this pin low sets the UART baud rate to 9,600. By default the 
  pin is high with a baud rate of 115,200.