Whenever we do a CM upgrade, we will always have one phone that receives VM but no MWL. It is usually several days later when the user realizes it. It is always a different extension. What causes this and what can we do to prevent this problem?
Long time my friend! Hope all is well.
This is very common when doing any significant changes on CUCM (like an upgrade). The way we usually mitigate these type of issues is to run a MWI Re-sync after the changes are done (usually the last step).
In Unity Connection go to Telephony Integrations - Phone System - Click the Run Button for Synchronize All MWIs on This Phone System.
This is available in Unity versions as well :)
Hope this helps!