How to fix Wi-Fi calling issues on iPhone SE (2nd generation)?

I put in a request for a call from Apple Support. I made sure to set my phone to not silence unknown callers, as I've had problems before not getting such calls. This time, I happened to notice the green bar at the top (not in phone app at the time) though I hadn't heard the call, but for the first time I noticed also that wifi-calling was turned off(!?!). As I live in a slight dip that has bad cellphone coverage, we have wifi-calling. Of course, the call dropped before I could get it. But wifi-calling turned off while Apple was calling, then turned back on!?!?!? I know it's a carrier specific issue, but I've had this problem with previous carriers too, and given that it was specific to Apple's call...Is there some way Apple can turn off wifi calling when it's coming from them? That'd explain why I have real trouble getting them by phone, but not via chat. That'd be infuriating if true. Has anyone had the same experience? iPhone SE 2, latest OS, Mint (T-Mobile).

[Re-Titled by Moderator]