I made a slight modification to the RF24Audio library to get the "call" button feature to work.
TMRh20 originally uses digital pin 6 on the Arduino as a way to visualize the audio output. Instead, I modified the output of pin 6 to output HIGH when audio was being transmitted and LOW when audio is not being transmitted. Pretty simple.
Please look for these lines (pictured below) in the RF24Audio library and make the modification as shown. It's easy enough to use CTRL+F "ledPin." Otherwise, download the RF24Audio.cpp file from the Wristwatch Walkie Talkie GitHub repository. https://github.com/hoilett/Arduino-Walkie-Talkie-Instructable/tree/master/Slightly_Modified_RF24Audio_File