Assuming you’re configuring the device as a SIP device as that’s the only protocol it supports. Your IOS version is very old, I recommend you to upgrade to something more current, like 17.12.4a.
Not sure if I understood what you meant. Can you please elaborate? If you can share the commands you use it would much easier to understand what you mean.
Upgrade the IOS to the version that introduced support for the 78XX series. This will allow you to find the phone type when configuring the voice register pool. If you don't upgrade, you'll need to use the fast track option. which is no longer necessary for the 78XX series with the latest versions. Upgrading to the latest version will provide the phone type you need under the voice register pool.
You can upgrade the IOS by downloading it from software.cisco.com, copying the file to the router flash, and modifying the boot command. However, to download the software, you need a CCO-ID with an assigned support contract that covers this router model.
Additionally, check the ROMMON versions.
Since you are currently on a version where smart licensing is not mandatory, please ensure you have all the licenses for both CME and UC on the smart account before upgrading to the latest version, where smart licensing is mandatory."
