Breech of contract

we have had two pay as you go - Top Up contracts with MTN for the past four years. On the 14-02-2015 and the 28-02-2015 we went into MTN to renew these contracts for a further two years. We got the My Choice-Top Up contract with 50mb usage limit. We were very specific about the usage limit as the contracts are for my children and we wanted no surprise bills. We have signed contracts which clearly state that they are Top Up contracts.<br> The first three months they were in place, we had to buy air time vouchers when the 50mb usage was up. on the 30th of June 2015 MTN took an extra R1206.34 from our account. We contacted them and were informed that the payment for July will be R8485.72. They investigated and said the data was used and that is what we owe. This should never have been able to happen, we made sure our contract was in place and had no reason to even think that in the fourth month MTN would change our contract. We have been paying what our bill should be by electronic transfer since the end of July(can submit proof of payment)<br> Why does my contract mean nothing, how can MTN change your contract and expect you to pay, who protects the client when MTN does this.