Recently performed an upgrade og cucm from 7.1.5 to 9.1. All phones were upgraded from 8.5.3 to 9.3.1 except for one site, this site is now having issues.
This site also has different phones then the other sites, a mixture of 7945 and 6921's. The issue is that these phones, for the most part, are not registering to their primary ucm. the phones boot, look like they try to register to the primary and then end up registering to the secondary. the phones that do register to the primary are also still running their original firmware (8.5.3)
I would focus on why the phones are not getting upgraded, first review the phone config to ensure it is not hard coded to a specific version, if that is not the case then use wireshark and/or phone console logs to review why things are not working as expected.
I think your phone model doesn't support direct upgrade from 8.5.3 to 9.3 and as you said that the remote site phone models are different from the other site then in that case please have a look for below link
I have found something strange, it turns out that the phones that are not registering to the publisher are also not pingable from the publisher. All of these phones and the publisher are in the same subnet, yet some are pingable and some are not.
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.