Terrible service

I was a customer for years with att. I called last August to cancel my phone subscription. I waited until my contract was up to avoid the early cancellation fees.
I had recieved a work phone and no longer needed the $150 bill every month. Everything went well when I cancelled or so I thought. I ended my auto pay and all was good. Eleven months later I receive a call from collections saying I owe $550 to ATT. Naturally I told them I wasn't paying until I knew why I had been charged. I called ATT support several times and was lead down a giant rabbit hole every time. The closest I came to some sort of answer was that I didn't cancel my phone, I only suspended it. Why would anyone suspend a phone and pay a ridiculous monthly rate. And why would I end my auto pay. Needless to say, I have been trying to get to the bottom of it still with no help. Every customer support person just shovels me off to the next. As far as I'm concerned ATT may have the worthy support I have ever seen. And they will not get a dime from me until someone explains to me how they messed up and suspended an account when I asked to CANCEL!!