We have a customer with a UC520 (8 user). They complain almost monthly that the time on the phones speeds up. Say the PC says "1:20" the phone will say "1:28". I have synced the UC to a local computer and assigned it as the NTP. This still happens.
Any ideas why? Is there a better time server to attach to for time?
*Clock is unsynchronized, stratum 16, no reference clock*
Statum 16 is way way down on the list of NTP servers. You are getting your time from the 16th server in the pool. I would choose a different pool or different server. You should be pulling your time from a stratum 8 server at maximum. BTW stratum 16 is the last server in the pool so it might not even be responding to your requests.
You know I overlooked something in your configuration. I would try setting up your UC500 to be a peer. I have run into issues like this with different IOS versions