TMobile is billing us under two different account numbers, and we cannot access the first one.

We're in a similar boat. we had just gotten our discounts ironed out with sprint as tenured customers for over 20 years and added a fourth line that was supposed to be free forever after 25$ activation, because it was a good deal for something we might want for my SO. My mom had submitted her form DD-214, they added the autopay discounts, senior for my GMA, loyalty, the whole nine. It took over a hundred bucks off of our bill, which was a relief.

then the switch was announced, and the fourth line , on its first billing cycle, cost 47$ or so. We spent hours getting that resolved with a refund and a guarantee it wouldn't happen again, but then came the second, then the third month, and on the fourth we just canceled the line. Every month there's a new issue with billing, device repair or replacement or software or other issues. Usually billing though. The last few months since the changeover completed, the issue is that my GMA is the primary account holder, but I pay the bill. I always kept track of the discounts and charges and services on the account, the leases on the devices, etc.  for two months it's prompted me to ask for full access from the primary, but when I go to request it through their little notification form, all I get is a "wires crossed" message. My GMA hasn't set up an id for T-Mobile and doesn't want to and in fact refuses to point blank, because "you always take care of all that, I don't need it, when do I look at it?" Also "you were able to do it through sprint so I don't see why I have to be involved." Yet T-Mobile won't even TALK to me, I've been on the account as long as my family has and they refuse to even send a verification code to my number when talking about my device and my line exclusively. 
Tmo, I never thought to love sprint, but then, my situation never had YOU in comparison. The reps are at least eager to fix the issue and sound enthusiastic, even if not usually helpful - the last seven-hour-long chat I had with them, I ended up coming to the right conclusion with almost no input from the agent - but the policies are AWFUL. I shouldn't have to talk to someone every month about my bill - and I should be able to have my bill in front of me when discussing it! Ridiculous!