T-mobile never stopped billing me even months after account closed!

I didn't realize so many were also having this same issue. I switched my number to Att after having service problems. Had been with TMobile for 5 years and suddenly my phone wasn't picking up bars. I requested to cancel EVERYTHING and I spoke specifically to the person on the phone, who I was transferred to and he (a tech here in the states) confirmed everything was cancelled as I requested. One more billing cycle would come out as I cancelled in the middle, but I was fine with that and was told that would be all I had to pay. In the meantime, I moved from an apartment to a house. I no longer was a customer and my bill had been paid, so I shouldn't have to worry about it, right? Wrong. I get a collections notice at my new address. I called and told them that this wouldn't be possible because I cancelled. I laid out why and they agreed. I had also not received notice of any kind from T-Mobile. Come to find out, they had sent a bill to my apartment address after I had already changed my mailing address with them, which is how the collection agency even had my new address. Talk to someone at T-Mobile, says something about "resolving the issue", but there was not a resolution. Since you're no longer a customer, they won't talk to you. I keep getting notices, it keeps messing with my credit, and I keep disputing on my report. It has been an uphill battle to get this taken care of. I will not pay for this. Sounds like it's done on purpose and I can't for the life of me figure out why. If anyone has dealt with same issue and wants to start a lawsuit, please let me know. I'd like damages to be awarded to those of us who've experienced unfair business practices by this company and take from them.