Worst customer service - cancellation of contract

I had a 24 month contract with Cell C which was due to end the end of April. I upgraded the contract sometime in March and subsequently decided to cancel the contract. The cell phone was returned in same condition as delivered and return receipt acknowledged by Cell C. I called in and enquired what the procedure was to have the upgrade reversed and cancel the contract. A case was created for reversal. Followed up numerous times since the phone has been returned. Call centre kept referring me back and forth to different departments within Cell C saying the RMA number is missing. Once the RMA number was located the matter still could not be resolved. I've spoken to a supervisor and he also couldn't resolve the matter. To date I have various ref numbers along with the RMA number and nothing still has been done for 2 months now. The contract was meant to be cancelled end of April and I'm pretty sure that I will have pay for the month of May. It's ridiculous and I will never refer anyone to Cell C. RMA = ***. Latest ref number: ***