I guess the obvious question stands out as, "Is your source NTP server stable?" Don't try to syncronize your CUCM to a MS Windows computer. While the UCOS will let you do this, it won't actually work well at all.
For best results, use a local Router/Switch as your NTP source and have that Router/Switch synced to a stratum 1 or 2 server on the Internet or a GPS based unit on the roof of your datacenter.
Remember, if you change your primary NTP source, be sure to get a new license from licensing@cisco.com.
-Steven
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