Billed for 4 months after cancelation request

I called in June 2015 to cancel my contract that was expiring in July, I even received a call from an agent asking me to upgrade and I explained that the contract was cancelled. I am still beinh billed4 months after the cancelation date and 5 fold the contract amount as apparently someone has bewn making calls from the number. The number has however not ever been in use by myslelf, I only got the contract for the cellphone. Their billing, cancelation and service department are shocking. When asked to speak to a manager I was told that all the managers where in a meeting and they could not even give me a time frame of when I would be able to speak to a manager