Handle hangup cause for ringing and callwt stop signals, REF 16384

The ringing and callwt stop signals received in voicemngr includes additional
information sent by asterisk about the hangup cause. If the
data=answered in mentioned signals then the call is answered by other
extension connected to the same line to which incoming call was proceeded.
Otherwise the signals are received with data=null what means it is a missed
call. Handle it properly in voicemngr, sending the corresponding information
to dectmngr about the reason of stop-ringing.