If the issue is reproducible, then a parallel capture from the IP phone as well as the primary callmanager server ( to which the ip phone is registered ) would prove if any TCP / Skinny / SIP messages are getting lost during transit.
Try registering with a diff server, check if only one cipc is affected or a lot of them, CPU / Memory usage on cucm when the issue occurs, apart from checking intermediate devices as suggested by Jaime.
I did what you mentioned. Since we have two cucm server (Pub and Sub) I've configured only one server on user cipc. So far, everything seems to be good. I will switch to the another server eventually and if the user cipc drop calls while connected to the second server, I will open RTMT and see on the logs. There is no error whatsoever on user interface and we have a lot more users on the same floor without any problem!
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.