Cell C and Their Service Sucks!

A few years ago my phone was ****** out of my bad and I went to the nearest Cell C shop at that stage to report it as such. I asked them to stop the phone and everything you supposedly should do if this ever happens to you. I never heard from them again regarding this matter. SIX years later I received a letter from attorneys to say that I have been handed over due to not paying my account! Needless to say I was shocked and really upset after finding out the contract had never been stopped as they had said. I ended up paying the outstanding balance as requested. After continuous payments I called to request a detailed balance only to see that I had now overpaid them for this contract. Its been over a year and countless mails have been sent requesting the funds to be paid back. Now, Cell C claims that I don't even show up on their system anymore and legal doesn't return any calls or even answer their phones so please tell what my next step should be? This is PATHETIC!