MTN bad service

Yes they are quick to say when you owe them money but if you need there assistance then they take there own sweet time they have been overcharging me know for the last 6 months now and still haven't corrected the problem my original contract is R279 but charging me like R519 a month which is not right I have been with MTN for mor then 15years and this how they treat there clients if they not going to sort it out soon then I will be canceling my contracts by them and I have like 9 contracts with MTN