Some ridiculous charge of $35.78 was sent to Collections. I reached a live chat rep over the weekend. They confirmed the amount and gave me a number to the collections agency which doesn't work! I have not been a customer for years. What a nasty little scam you guys are running. Now the live chat feature is magically unavailable.
Once T-Mobile sends it to a Collections Agency, what that Collections Agency does with it is beyond their control. If said Collections Agency sells it to another, T-Mobile can't give you that new place's contact info.
Your best bet is to get your Credit Report and find out who has it and contact them that way.
You can still try within T-Mobile. I'd suggest T-Force via Social Media for that - Contact Us -- but, they may not have any additional info on who has your Collections Account.
Manage your T-Mobile account