Incorrect billing

Wow mtn didnt realise you could get this so wrong.,my hushand moved me to an mtn sky contract in dec (unlimited calls and 30 g of data) . Today we get a bill for 2368. How is that possible??? It should be 499 as per contract. The account number is a***. It had been a series of terrible customer care. Last month mtn migrated the same contract to something else then we had to call to move it back to sky. Pls sort your lousy service out