Over charged and told won't be refunded

On 27-01-2015 I took a new contract with Cell C for iPhone6, on the advert it said a complete total will be R600., on the 25-02-15 I was billed billed R1924.82 and when I went to query i was told it's the packge I then told the Agent that took the contract which was still on display, she said she is a trainer and will have to call Cell C, I requested the manager and was advised she is on lunch. I went back again on 21-03 I was told I won't be refunded because it's my fault and the agent didn't explain how it was going to work. I took the phone based on the advert now I'm being overcharged, what kind of service is this letting trainees work without supervision and blame the client. I want my refund and get the service I pay for.