Cell C Not Providing Feedback

I have a Cell C data account with a modem. I have been over-billed by R450 over the last 2 months. They claim that I subscribed to services on the internet hence the extra charges. I know that I did not subscribe, since I am a fully computer literate, IT Specialist. I have never subscribed to anything using a cell phone number in my life. They said that somebody will contact me regarding a refund, but I just wait for weeks with no contact. Indirectly, I am their bank and I have loaned them R450. I should get interest when they decide to refund me. That is the way I see it and that is how the economy works.