I have been having many, many issues with my callmanager and Unity servers becoming unsynchronized with the NTP server. I have a TAC case open and they said "
If this is also happening for CUCM cluster, I suggest changing the NTP server. Cisco recommends using a Linux based NTP server since it is more stable. A NTP server unsync could bring serious issues like a replication problem which can extremely affect the production."
I currently am pointing my CUCM and Unity Publisher servers to Google. I'm not sure if they are Linux based or not. Is there a good, Linux based NTP server that you use? I am in North America.