This is a very, very old phone. There is another problem, these phones use a Cisco-inherent protocol.
If you want to have full functionality, you have to patch Asterisk, but this is only for advanced users.
https://usecallmanager.nz/documentation-overview.html
For example, the BLF-buttons won’t work without the patch. You also have to understand the structure of the Cisco-phone-config file…and you have to edit it manually.
I have been using a dozen of Cisco 8961s that way. I don’t use Endpointmanager. The EPM’s phone support is always incomplete, especially with Ciscos.
A second option, instead of patching Asterisk, is to use the SCCP module. Don’t know how reliable it is…