Charged after cancelling

So I changed my prepaid account to post-paid T-mobile service since last year August as I had to leave from USA to overseas for a month and needed an international service. First the representatives mislead me that I will be charged one time pay for out of country service only to come back to USA and charged much more than I was told. Also I wanted to transfer the same phone number to post paid which they said will only take few hours. It didn’t happen and my flight is in few hours, I keep calling them and they say it will happen within 24hrs. By the time I get to the other country I still don’t have service. I spend weeks calling them to update my phone number. When I come back to USA, they have charged me for all the times I have called them saying since I was calling USA T-Mobile service number so I am being charged and how I should’ve called international t-mobile service number. We had an argument for all of their misleading information first about international charge and then not updating my phone number which gave me so much stress overseas to keep calling them and then being charged for it. They tell me they will cancel one of the big charge since I wasn’t told about it. So I become fine only to keep getting texts again that I owe this much amount. I call them again and they say no it was never canceled and I have to pay. I tell them the previous time I called they canceled that amount due and was told I wouldn’t be charged for it. This customer representative doesn’t agree. I just didn’t want to fight anymore so I just pay all and cancel the service and they tell me my balance is 0. I will be never be charged again as this account is canceled and balance is 0 now. I switch to prepaid, but now collection agency is keep calling for balance. I call back t-mobile post paid account, they tell me yes I have balance due and extra cost also due from collection agency. I am so sick and tired from this scam company keep charging me when I have canceled the account for months and never used the service and had also paid for remaining balance before closing that account. Now they tell me I have more balance due. I am at point of switching to another service. Their internet service already is crap. They keep stealing money from me. What should I do? Should I sue them or what? Why they are keep charging me bills?