T-Mobile Sent me to collections, because of an error they made during a Transfer of financial responsibility Internet

 About 3 months ago I did a Transfer of financial responsibility through T-Mobile to another party..When I did the transfer the account balance was $0.  I even called back twice to verify the transfer was complete Apparently before the transfer was completed T-Mobile said their was an "invaild account" Because of this "Invaild Account", T-Mobile canceled and applied an ETF fee for 2 lines and never informed me about it.. Untill after the account was already in collections

  Because of that glitch on THEIR (Tmobile’s) end, they charged me an ETF instead of the other party 
Because of that I am now in collections for $500 plus, and nether T-Mobile or the collections agency will listen to me. All they tell me is “Pay or else this is going on your credit”