Allow to answer, reject or terminate a call.
Answering or rejecting a call can only be done on an incoming call.
Terminating a call can only be done on active, dialing or alerting call.
The Voice object lists the Call objects, which provides status of each call.
+CIND call indicator is set if at least one of the call is active.
+CIND callsetup indicator is set if one of the call is in ringing in or out
or dialing state.
The Modem object provides the own number (used by +CNUM), the network
service availability and signal strength (used for the +CIND).
+CIND indicators can be activated/deactivated using AT+BIA except for
call, callsetup and callheld indicators which should always reported.
All indicators are enabled on connection.
