I have been a T-mobile customer for years, at least 10, about a year ago while walking on the mall, one guy in one of those t-mobile stands told me he could lower my bill, but I needed to get a new number. I did, the problem is that he did not cancelled my old line, and I got two bills the next month. I went over this issue with customer support for months, and they never fixed the problem, they wanted me to pay no matter what. I said no way, I am not paying.
I owed them a month, they wanted 2, for the second month I was already using my new line.
What happened? well they sent that to some collector to whom I also said no way, I am paying only a month.
I don't know what happend, they stopped calling me, and they never reported me to any credit bureau.
Instead they could have taken 1 month, as it should be.