Lync Monitoring reporting showing Caller and Callee connectivity as 'Failed'

The call below was between an external user and an internal user.  Would it imply a voice path couldn't be established?  i don't think the call failed as the diagnostic ID is 51004 (Action initiated by user) and there are stats for the audio streams

immediately after this call, the same two users tried it again and this time it showed as i would expect it (as below)

What i'm trying to understand is, what would cause the Caller/Callee connectivity to show as failed and does it mean that no voice path between the endpoints could be established?