So, I’m not sure how much overlap there is in our solutions here, but your groups theory made me poke around on that side of things.
The group I was pulling from Active Directory had nothing defined in the “Show in Contact Manager Groups” and “Viewable Contact Manager Groups”, under the “Contact Manager” tab. I assigned “All Internal Groups” for “Show in Contact Manager Groups” and “All Public Groups” in “Viewable Contact Manager Groups”, applied the settings, and now the Desktop Phone loads without issue.
Thanks for the shove in the right direction! Would have never thought to look there on my own.