Generally good in urban areas, can be hit or miss in more rural spots. My wife used to be on AT&T and sometimes she had better service. The frequency bands that TMo uses also don’t penetrate buildings as well, so inside a huge office building or when underground you might have weaker service and slower data speeds. Worth testing out in places you go to frequently before you make the jump.

Modernish phones should support wifi calling on their network. At least with actual T-Mobile plans, not sure about Mint. That helps at home or at work.