Product: Debt collection

I had been a customer of XXXX for XXXX years when I decided to switch to a different carrier as XXXXXXXX XXXX customer service ( which used to be excellent ) had become horrible. For XXXX years, I always paid my bill on time every month, never once was I in default. I cancelled my plan with XXXX on XX/XX/XXXX and switched my phone number to the new carrier. I called XXXX to do this and explained the situation to the woman over the phone and as a courtesy because of the terrible experiences I had gone through all month long of XXXX, she credited the remaining balance due on my bill, which was around {$52.00}. In XXXX, I started receiving bills from XXXX stating that I owed them over {$200.00}. There was no explanation on these bills, just that I owed them money. I tried to log into my account to view it, but my account was locked since I was no longer a customer. I never once received a phone call or an email from XXXX. I only received these monthly statements that told me nothing other than the amount due. I spoke with a representative at that time and I explained the situation to her, she said that she would escalate the situation to her manager ( she was literally not able to tell me anything about the balance ) and that she would call me back in 48 hours at XXXX EST. Never heard back from this representative. Then, in XX/XX/XXXX, I received a call from a collection agency stating that the account was in collections and they were trying to get payment. I angrily explained the situation as I DID NOT owe anything to XXXX. My balance was cleared on XX/XX/XXXX. I then spoke with a different representative at XXXX and she kept telling me that the balance was from a hot spot device that was never returned, from XXXX!! I explained to her a few times that that makes no sense as I did return the device and if she looked back at my account from XXXX, she would see the credit from the return. She, again, told me that she would escalate the situation to a manager and I could call back in 3 days to get an answer. At this time, I was beyond frustrated as I did NOT owe them anything and filed a complaint with the XXXX. After that, a representative called me to say that it was not the hot spot, but some router that they had given me years ago that I never returned. I assure you, I never purchased this router and I don't know what they were talking about. Then, she proceeded to tell me that because I switched over my voice line, the other lines on my account ( from the time I very briefly had the hot spot device ) started accumulating charges. EVERY SINGLE MONTH, those other lines were {$0.00} - I thought it was clear that when I cancelled my plan with them, all lines were cancelled. Then, they had the nerve to send me to collections because of their screw up!! I have never once been sent to collections for any debt and now, thanks to XXXX, I have a collection dent on my credit report. The representative agreed to credit my account and remove it from collections, but that is still not acceptable that I even had to go through this due to their negligence and incompetence. Never again, will I ever do business with them. They are completely unethical.