Telkom over charging me on my contract

I took up a Telkom Mobile's R499 Sim only deal in March which included airtime, SMS' and data. I was expecting to be debited R499 but I am currently being billed R658.41 every month. I was a bit sceptical about signing up with Telkom as I heard some funky stories about them but wanted to give them a benefit of a doubt but I have been left with an egg on my face. Trying to call their accounts query call centre my call has on 3 occassions been cut. Please help.