Is this on-premise IM&P or SaaS WebEx Messenger for a backend?
For on-premise this is done server-side between CUCM and the IM&P cluster. For this to work the end user must be associated to the line appearance of the DN on the device making the call, not the End User Configuration page. Has this been done on that specific line appearance? If yes, does CUCM send the SIP PUBLISH event for that call? You'll need the SDI traces for this. If yes, does the SIP Proxy on CUP receive the event and turn it into an event for the Enterprise Presence Engine?
As you can see this quickly turns into a log review session. Honestly though, it's almost always a misconfiguration in CUCM causing this.
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