Billing Issue - Collections

I moved into a new house and wanted to have service at both houses for a few weeks. Xfinity customer service told me not to do a transfer of service, and they would waive the early termination on the old house since i was adding service at the new house. I received a bill for $140 weeks later after cancelling the old service. I called Xfinity and they said that was an error and they would credit my account. 2 months later i got a call from a debt collector saying Xfinity had sent my account to collections. I called Xfinity and they apologized and said they would let the debt collector know it wasnt a valid charge and promised they would take care of it. 3 months later it showed up on my credit report (now i am really mad), and i called Xfinity again. They said they would fix it, let the debt collector know it was not valid, and remove it from my credit report - and call me back in 3-5 business days to confirm. Three weeks later, i still have not got a call back. the debt collector reached out again today and said Xfinity continues to say its a valid charge. i called Xfinity today and they said they would take care of it, then transfer me to billing - they actually transferred me to a voicemail to someone in Xfinity business. I dont know what to do - i am considering hiring an attorney to file a lawsuit? how else can i find someone at Xfinity to help me. Has anyone had success filing a lawsuit in order to get someone to help you? i am in tears...