Frustrating to say the least. I don't want to think about how many hours I spent yesterday dealing with this issue. I even hooked to my mac and performed a recovery on the phone. Still made no difference. I do have some good news for you though. Yesterday, one of the Apple technicians escalated my issue to a senior advisor who said he would take over my case to the finish. He opened a ticket with engineering and I had very little faith anything would come of it until next week.
I woke up this morning, and tried it again first thing. No dice. I tried again after i got out of the shower at about 8 AM and VOILA! It worked! I don't think it even asked me for the last 4 of my social that time. I honestly can't remember since I was so excited it went through. So I guess either something was just broke yesterday or the engineer did something this morning to free up my phone to register with tmobile. You should tell them that you are aware of at least one other person that had the issue that had to be escalated to an engineer. Maybe they can get you escalated although I am not sure if they have any personnel that could escalate that on the weekends.
Good luck, please let me know if you get it up and running. I am curious if mine was a fluke or if there is something larger at play. Just curious, was your previous phone on eSim or on a regular sim? I have this nagging feeling that it had something to do with me being on eSim before or something.