The following functions are used to make and receive voice calls:
•gsmAnswer
Answers an incoming voice call.
•gsmHangup
Terminates an active voice call.
•gsmIncomingCall
Checks for incoming voice calls.
•gsmMakeCall
Makes a voice call.
•gsmMakeCallX
Makes a voice call with extended capabilities.
•gsmOffHook
Returns whether a voice session is active.
