1. CUCM or CM Telephony service getting restarted.
2. UCCX or any of its desktop related services getting restatred. Failover\Failback happening in case of HA system.
3. Network connectivity issues.
To find out the exact reason for failure, you need to check the logs from both the UCCXand CAD side.
And also if you login into UCCX serviceability page and navigate to control center, you could see since how long each of the UCCX services are up and running, try to see which is the service running from least time, which will give you an indication of this service restart.
CAD logs path,
UCCX logs can be downloaded from the RTMT tool, please see the MIVR, MCVD logs in this time duration.
Hope it helps.
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