SBV3202 & Xfinity Voice in 2023

I recently tried to replace our rented DOCSIS modem with an Arris SBV3202. We don't actually have Xfinity Internet, just TV & voice -- after the swap, and after the Xfinity rep updated our MAC address, the modem said it had working Internet but not voice. The Xfinity rep was very friendly and tried hard to be helpful but ultimately didn't know what could cause the issue.

Any thoughts out here? Have things changed such that this hardware is too old? The rep wanted to schedule a service call, but it's pretty hard for me to see that being the right answer (at least not to start with).