I recently changed from the major carriers to JMP.chat (and now USMobile) because my carrier randomly decided a Pixel5 can't work on their network. But if I buy a Pixel 5 from them, it works.
JMP.chat pipes your SMS into XMPP, which is awesome, for $5/mo, including phone calls. I just use USMobile for a data connection now.
Yes, but if a phone isn't on AT&Ts whitelist you can't even get a data connection. It's bs for sure (the phone can use the same bands), but it means you can't register it with AT&T, and I'm not going back on anything like sprint.