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If enabled, the PBX will send a "SIP 183 Progress" or it's equivalent to digital channels where supported. Some providers require this to keep the channel up while it's waiting to be answered.
Mark Answered Elsewhere
This parameter controls whether or not a 'missed call' notification shows on a phone if a ring group call is not answered. Provided it's supported by the ringing device, when 'answered elsewhere' is sent to ring group devices that were not answered, the missed call notification is suppressed
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