The call reference ID associates the video call entry and voice
call entry of the same endpoint. An audio-only call may or may
not have a valid call reference ID (i.e. value greater than
zero), but in both cases, there will not be a video call entry
associated with it.
For a video call, the video-specific information is stored in a
call entry in cVideoSessionActive of CISCO-VIDEO-SESSION-MIB, in
which the call reference ID is also identified.