MicrosoftSteven Hall This is likely due to an issue with the 3PIP gateway that these legacy devices use to connect to Teams. In my experience, Microsoft is not fixing any bugs with this service so I would not recommend pursuing this issue any further as your efforts are likely to be wasted.
As announced in this blog post, the 3PIP gateway is scheduled to be shut down on July 31, 2023. It will be replaced with a SIP Gateway service which will allow you to continue using these legacy phones for "https://www.microsoft.com/en-us/microsoft-365/blog/2020/08/03/take-communnext-level-microsoft-teams" which "may include inbound & outbound calls, call transfer, dial in/out to meetings, device-based do not disturb and voicemail indicators". Note that you will be required to reconfigure these phones into their OpenSIP mode to connect to this new service.
