Incorrect billing and no follow up on request

Vox administration of accounts is bad. I had a service for internet with Vox Telecommunications after cancelling my services I got charged a penalty for which I had no problem with and I made an arrangement to pay it for which I did. However I was further charged for a router which I indicated that Vox can come and pick up their router after struggling with Vox picking up their router I delivered it to one of their branches here in Bloemfontein and have proof of delievery however they are still sending me letter of demands even when I send them all proof of payments and delivery of their router.