Hi,
By default every phone will send a keepalive every 30 seconds and will receive a keepalive ack from primary CUCM. If the phone misses 3 keepalive acks (90 seconds) the TCP connection will be reset which can cause it to unregister and same thing is CUCM misses 2 keepalives (60 seconds). So first thing is to check from callmanager on the reason for device unregistered (you will see stationinit , keepalive timeout reason cdoe 13 etc). If you deterine the issue being caused because of the keepalive losses, then the next thing is to get packet captures both from callmanager and the Phone expeirencing the issue to make sure if these are sending keepalives/acks as they should and are getting lost in the network or the phoen itself stops to send keepalives.
Thanks,
Lakshmi
