Well, it's way past January 2021 and I have been using my Pro1 on T-mobile just fine, and now, while the Pro1 is being repaired, I am using my Pinephone just fine on T-mobile. This is prepaid-- I can't speak to post-paid.
From what I can tell, AT&T is most dead set on it being an approved phone. Verizon is somewhere in-between. As long as a phone has VoLTE and a SIM muled in an approved Verizon device, it can go on the network, but then, for some people, things will stop working, while others seem to have good success. T-Mobile seems only to care if it is VoLTE although, chances