Most likely it is a provider problem if the system had no problems with this for a long time AND there were no changes made on the system which could be a problem.
Check with monitor if the correct number is send to the line, if that is correct then call the provider to investigate the problem.
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Call Avaya, costs only €400 per hour, you must buy at least five hours.
But then you can talk to a REAL expert....
Now that is what i call support.