Hi there
Is this happening only for 8851 phones. Please post the console logs from the phones during the event. If this affects on a site mostly it could be WAN related issue. Try a continuous ping from the CUCM to the ip phone.
Also please check the Cisco Bug: CSCus85518 if you are hitting this.
https://bst.cloudapps.cisco.com/bugsearch/bug/%20CSCus85518
88XX phone can't register if more than 5 lines in phone button template
Symptom:
8841 or other 88XX phone model that doesn't support a Key Expansion Module will show "Unprovisioned" or "Telephony Service Unavailable" and fail to register to Unified Communications Manager (UCM).
Conditions:
This issue only occurs if the Phone Button Template assigned to the device in UCM has more than 5 lines assigned to it. The 8841 and 8811 phones only support a maximum of 5 lines and will reject the xml configuration file from UCM if more than 5 lines are present.
Workaround:
Use a Phone Button Template in UCM that is based on one of the Standard 8841 or 8811 which will not allow more than 5 lines to be configured.
Further Problem Description:
Any 88XX phone running 10.2(2) or later will display "Unprovisioned" when no lines have been configured in UCM. This defect causes the phone to stop parsing the config file before it detects the configured line. Phone firmware older than 10.2(2) will show "Telephony Service Unavailable".
Look for these lines in the phone's console logs to verify this is the problem you are facing.
9024 ERR 15:13:11.916351 Jan 5 15:13:11 JAVA: configmgr MQThread|cip.xml.XmlParser: - XML Parser Exception: Element '/device/MissedCallLoggingOption' : String too long: length is 7, max length = 5 (line=225)
Hope this helps !!!
Cheers
Rath!
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