The instructions for customizing a 7965 phone are at the link below. The guide is for CUCM version 9.0, which is the last version for which Cisco published a Phone Administration Guide for the 7900 series phones. But the steps still apply to current versions.
If you're sufficiently in the past, you could try and locate the Cisco IP Phone Designer application.
There are URIs that it used to push a ringtone and wallpaper, however I'd never figured out how to use them myself.
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