Overcharged on contract - no response to queries

After almost 12 years of having our business contracted with MTN and almost 2 of these years on a 2gb data package and paying an average of R400 - R500 for an average of 2.5 - 2.8gb, we recieved a bill of over R1500 for 3.5gb usage! I phoned to query, was told that a data validation would be sent for the last 3 months and recieved a ref no. a week later - nothing recieved and no answer on the phone (***) I sent an email, another ref no, but still no response AND the money was debited! sent another e mail today, yet another ref no, but don't have any faith in getting a response. the cherry on the cake? we are being smsed specials for 20GB bundles for R1200 (less than we are billed for 1.5GB over our bundle)<br> Basically, we are treating this as theft. MTN is unable/unwilling to try and rectify their mistakes and refuses to explain.