Not adhering to contract agreement

I took out a contract with MTN so they I can receive 300min monthly. Every month I have no minutes and have to pay rates for top up. I went to a MTN shop to sort this out and they said the dispute can take up to another 2 weeks to sort out. I have signed a contract confirming my agreement, yet MTN is not holding up to their part of the contract. How can they not sort this out immediately, why do I have to continue to pay these high rates and yet it is meant to be within my contract?