Billing Dispute - Mistaken Identity

My husband and I have a Cox internet account that is all paid up. However, we recently received a collection notice from Cox for an account from our old apartment complex. It was under my husband's name, so I double-checked that our old internet account (a different account number) was all paid-up and it was. 

I asked the collections agency to verify the debt, and they mailed me an old Cox statement. It was addressed to someone with my husband's first and last name (a very common name), and in the same apartment complex, but a different apartment number. This was a huge complex, so it doesn't surprise me that someone with my husband's same name was also living in that complex at the time...and apparently not paying their Cox bill!

I wrote the collections agency a letter explaining the situation, even including a copy of our old Cox statement, our old lease, etc. They continue sending the collections letters to us.

Does anyone know: how can I obtain a letter (or some sort of documentation) from Cox that clarifies the situation? I think I would need something along the lines of a letter that states Cox provided accounts to two different "Mark Smiths" at apartments ____ and ______ during 2016 - 2018?

Thanks very much!