Thanks. Can you send me the link on the heartbeat between IP Phone and CUCM? Also, I can see in status message in network statistics as "TCP-Timeout"; can you let me know about more status messages?

Sorry, should have said keep alives instead of heartbeats.

see this thread: http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Unified%20Communications%20and%20Video&topic=IP%20Telephony&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.2cbf3f50

Are all the phones falling into "registering" mode when they lose connection?

Yes. All phones get hard booted

Check the Switch they are on first to make sure you are not getting some kind of Power problem before you go and trace out the whole network for keep alives, etc. If it's a simple POE problem with the switch, that is the problem.

Turn on SNMP on the switch they are plugged into and log the traps. If it is a POE issue, you should see either the switch rebooting or POE messages, etc.

If the phones have power bricks, then it might be different. bricks will not typically cause hard reboots of IP phones.