Please help me cancel my account!

My last phone usage was in September of 2021, I went ahead and paid for October because I had a "free month" elsewhere. I went into store & closed it. Apparently he suspended it not cancelled. I got a notice in Dec. & paid for November (just to have the whole thing behind me). Now I am getting a bill March of 2022 stating $267.10 will be reported to my credit bureau. I have called customer service 3 times over an hour per call, being bounced everywhere. My hands are tied because I don't have the phone number anymore & so can't access it. Then I get messages stating system error. Though it is closed now (apparently confirmed by an agent, how do I truly know since I can't access my account) And there is a lingering balance of $267.10.  I am a disabled veteran dealing with severe panic attacks (also PTSD), sending me into the ER. Please any help is appreciated. PLEASE!