My phone (Pixel 3a) has been dropping longer calls when using wi-fi calling over T-Mobile Home Internet. It’s happened about four times now, including once to T-Mobile support. Once, after it dropped, it wouldn’t allow me to call back (just hung at “Dialing”) or receive voice mail until I switched off wi-fi. I’ve set it to “Call over mobile network” for now, which means it will prefer the cellular network, falling back to wi-fi only if cellular fails. The only problem with that is I now only have 2GB mobile data (hard capped), and I’m not sure if VoLTE, visual voicemail, or SMS use data nowadays.