I am having the same issue today and what is really annoying is when I enter the extension number nothing happens.
I was using the Atlanta server --- so I switched back to the Montreal server and now when I enter the extension number it now works but no ring-back when placing the call to this specific client. Now I know that my client very recently switched all their lines from Bell POTS to Rogers VoIP so maybe there is a tie-in to that -- will have to give it some time and see if this sorts out before end of January.
Perhaps there are numerous changes taking place with people switching from Bell POTS to VoIP and the "Telephone System" is having trouble adjusting to all these changes.--
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