Product: Debt collection

Upon discontinuing my XXXX  internet service in XXXX of this year ( XXXX ) I received a notice that there was a piece of XXXX equipment ( an XXXX  wifi base ) that needed to be returned. 
I had returned all of XXXX XXXX  equipment two years previously when I decided to save money by purchasing my own equipment and no longer rent theirs. 
I have had numerous notices, with each I contacted the company to attempt a resolution. 
Without responding to the information I've provided, or providing me with someone to discuss the disputed equipment they turned my account over to a collection agency. 
I tried repeated to communicate with them but received nothing but more notices. 
This has been a frustrating and time consuming issue that now threatens my credit. I don't know what else to do. 

Here is the sequence of communication I have had with XXXX  since I discontinued service with them in XXXX of this year ( XXXX ). 

XX/XX/XXXX I received by letter a request to return an XXXX base or I would be charged {$190.00} for unreturned equipment. 
XXXX I called the number given on the form and told a XXXX I had already returned this equipment..he told me " if I've previously returned the equipment to disregard the envelope ''. 

Upon reflection I didn't think XXXX actually understood the issue ( since I returned the equipment two years ago. ) I called back. 
XX/XX/XXXX XXXX I spoke with an XXXX and restated the issue and that I had already returned this a couple of years ago ... he stated " it's always protocol to send a box but he verified I had no XXXX equipment to return. 

XX/XX/XXXX I received another letter stating that I had unreturned equipment I spoke with XXXX who initiated a search at the warehouse for the disputed XXXX base and it was not to be found. He gave the email contact to dispute this " unreturned equipment ". I emailed them. 

On XX/XX/XXXX XXXX XXXX left a voice mail on my cel stating that the piece of equipment has not been returned and promised to contact me at a later date. ( She never did ). I contacted XXXX  at the phone number she ( XXXX XXXX left... it was the same and only number to contact XXXX So I spoke with a XXXX ( XXXX ) and we did the same search of the warehouse and not surprisingly the equipment was not found. 
Not surprisingly because everyone at XXXX kept doing the same thing with the same tools and kept coming up with no equipment. 

So I used the only contact I had besides the phone number that was not getting any results and sent the receipt from the XXXX units I purchased to replace the " rented ones ''. This was to show that I had no reason to keep the disputed XXXX base. 
Furthermore XXXX XXXXs monthly billing also shows that as of XX/XX/XXXX they stopped billing me for rented units- which I contend is because they were returned. Certainly they would have initiated this issue of unreturned equipment in XXXX. 

The summary of the issue- In XXXX I purchased my own equipment and returned XXXX XXXX to save me money. When I discontinued my service in summer of XXXX they said I had a piece of their equipment and needed to return it. Each time they initiated contact with me I responded in the only way they gave me to try make a good faith effort to resolve the issue. Each time they responded by saying simply we don't show this equipment returned- therefore you owe us {$190.00}. 


I have copies of all correspondence and the receipt from my equipment I purchased prior to returning ALL of theirs. 

Again I dispute that I owe them anything.