This will enter or exit listening mode, which opens a Serial connection to get Cellular information such as the IMEI or CCID over USB.
// SYNTAX - enter listening mode Cellular.listen();
Listening mode blocks application code. Advanced cases that use multithreading, interrupts, or system events have the ability to continue to execute application code while in listening mode, and may wish to then exit listening mode, such as after a timeout. Listening mode is stopped using this syntax:
// SYNTAX - exit listening mode Cellular.listen(false);