I have a Pixel 4a 5G (bought through TMobile) with wifi calling enabled (prefer wifi). Sometimes, I cannot hear audio from incoming callers, but they can hear me. If I disable wifi (forcing the call to cellular) the audio problem is fixed and I can communicate. Once audio is working on a call, I can re-enable wifi and audio continues working.
Multiple people are having this problem, both with Android 11 and now with Android 12. See this bug:
Most of the reports are from people with T-Mobile service.