MidLand Funing LLC is on my credit report record

Please everyone i need your opinion on this. I lived in Florida, and at May 2007 my daughter make a international call with T Mobile on her cell phone and the bill goes up to $7000, and i am the line owner, so Tmobile go after me. T-mobile disconnect all our cell phone until we have to pay that amount. I call them asking if they can low that amount because that is way to much for me, after speak to the supervisor she told me i can just pay the $5175 and that all she can do or else they going to sue us. After hearing that i pay them the full amount using my sister credit card and mine. After that all i receipt is the paper which write in hand writing that everything is pay and settle on March 2008, without T mobile logo, sign or anything. And thats stupid of me, i thought that was it, and im done with them, i thought everything was settle, but no at July 2012 i receive a letter from Midland Fund LLC saying that T mobile is selling this case to them and that i still owe the amount $2010.00 with T Mobile that need to be pay, I call T mobile and they say dont worry about it, they will take care of it. And so i thought that was it. Yesterday March, 24, 2013 when i try to go apply for home loan mortgage the lady told me that in my credit report i owe $2101.00 with MIDLAND FUND and need to be pay in full, or else i wont be able to closing the house and final approval. I call T mobile again and the guy told me that they dont have any of my record and that i owe $2159.00 to Midland Fund and need to be pay. I told them i already pay them 5 year ago with the amount that we agree to settle and i still got receipt , and the guy told me that they no longer have anything to do with this account and that i need to contact MidLand Fund. I call MidLand Fund and told them about this and they told me to call T mobile and have them clear this out but i told them T mobile told me to call you guys and they say they dont have any record about this account anymore. And the guy in Midland fund told me that they have send me 4 letters to my house to let me know that i owe them that $$$ (but the only letter i got from them is at July 2012 when they pick up this case), and i need to pay that amount because there is nothing they cant do, then he hang off. Guys what do i need to do in this why would MidLand Fund pick up this case 5 year later? what can i do about this? Tmobile and Midland FUnd is pointing their finger at each other about this. all i have is the statement that T Mobile charged my sister credit card $3105.00 and my credit card $2070.00 at same day at 2008. i also got the hand writing receipt after they took the payment and fax me back i dont think this hand writing receipt from T mobile is gonna help me. As i read everyone comment in here i saw a lot ppl got sue by them but so far i dont got sue yet because i think they not gonna win this case or they don't have proof. I also have send Midland Fund a validation letter yesterday, What is my next step should be? please everyone please help me with this.