Add hangupcause for stop ringing signals towards voicemngr, REF 16384

Grzegorz Slujarequested to merge
fix_16384 into asterisk_chan_voicemngr_rdkb 
Add hangupcause for stop ringing signals towards voicemngr, REF 16384

The information 'hangupcause' will let voicemngr and dectmngr
differentiate if the incoming call is released by the caller or it has
been answered on other extension mapped to the same line.

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