OK, I have a new-to-me OnePlus 6T, the T-Mobile version, now carrier unlocked. It’s running Android 11. I put the USMobile SIM that had been not working in the SFOS Xperia 10 II in there and it started working in short order. After verifying function for about a day, I’ve moved the SIM into the 10 II, and after a minute or so I got network connection and 4g voice working. For now it’s still running.
So I’ve tried 3 brand-new SIMs in the Sony 10 II running SFOS before putting them in another device, 2 T-Mo and 1 USMobile (running on T-Mo network). None of them started working in the SFOS device before being in another phone. The first T-Mo SIM worked for three or 4 days on Sailfish after a short time in a pinephone before getting “denied” permanently in the 10 II. The second T-Mo SIM is still in the pinephone, still working, and hasn’t returned to the SFOS device due to my paranoia. The USMobile SIM has only been working with SFOS for a half hour.
Update: sometime overnight, the US Mobile SIM got “denied” in the SFOS Xperia 10 II. I put it back in the Oneplus 6, and it came back to life. Hmmm.