Mtn
MTN doesnt want to tell me what happend to /-gig

I had with MTN a 500meg and modem contract costing R69.June I decided to upgrade
My contract to a 1gig which would then cost me R79.The first month june went ok I got
My gig then July I didn't check on the day but noticed that the next day I had around
400meg left, should have carried over about 300meg, Hadn't been surfing a lot or anything. I, m missing 900 meg and MTN refuses to give me details of the 3 days involved. I just wan't to make sure they gave me my gig and not 500meg, they just fob me off. What am I supposed to do???