Convergent Outsourcing WA review: Owing them $120 for a t-mobile bill I already paid!

They contacted me and said I needed to pay $120 for a T-mobile bill or upload the canceled check that T-mobile had cashed. I uploaded my check I sent T-mobile 50 times. But the check I wrote and was cashed was for $146.24. My bill was for $124. I also sent them documentation from the Federal Reserves banking system showing my check went through to T-mobile's account.

They told me T-mobile won't except official documentation from the Federal reserves. ?

T-mobile owes me at least $23. They told me they would settle for $90. Why? This is WRONG! How do you stop this?