Incorrect billing by Cell C

I'm so irate with Cell C! In Nov 2014 my 2gig internet contract of R119 came to an end. I received a statement from Cell C in Dec 2014 with R279 to be debited. I gave them a call (Ref no: ***) to say that I did not want to renew the contract but I ended up upgrading & took out a 2gig contract for R99. They said they would credit me R180 & I'm still waiting for it even now. I made calls again regarding the R180 on the 17th of Dec (Ref no: ***). On the 19th of Jan 2015 (Ref no: ***) I made a query on a statement with a debit order amount of R146.90 instead of R99. The debit order went through nonetheless. Please sort this out as I'm tired of making calls in vain