Hi Nik / world
I have exactly the same problem.
This happens a few times a week. The ringing only occurs when I'm already in a meeting, and does not stop, even after that call has ended. The only way to stop this is to completely end the running Teams application from the system tray.
This happens both with Bluetooth headphones or the native laptop speakers. Like original poster, the ringing also carries over when changing audio devices whilst the issue occurs (on headphones > Issue occurs > Disconnect headphones > Issue continues on
speakers) and visa versa.
I have been having this issue repeatedly for several months, so a close app / restart of computer doesn't resolve the issue.
I have cleared the caches as outlined by Nik, but the issue is still apparent.
Regards
Neil