Refusal to Migrate Package

I took out contract in November. The agent explained that the contract was a top up package, After the airtime that was given to me I then attempted to recharge and I coul not recharge the account. I then called customer service and was told there is no way I can recharge the phone my phone. That is when i realised the phone is not on top up package. I have aclled Cell C numerous times to change to Top Up. Today I was told I can change to top up but I would need to pay R604. Why must I pay if this was not my fault in the first place. I want to cancel this contract asap. People warned us about Cell C. I ported from MTN to Cell C and this is my biggest regret.