packet switched data calls

I received a bill today by email as i normally do and I've noticed ive been billed over 600 for packet switched data calls.This is utterly a breach of contract. Ive been on hold and no answer on 808. Ive had my mtn contract for years and i am well aware of what package im on and how billing works so if someone tells me otherwise this is not acceptable.im actually engaging in litigation as this is a complete violation of the CPA.i will not pay a cent this month and will cancel my debit order of i do not get feedback.A contract cannot change in the blink of an eye without informing a customer.Absolutely disgusted.i have wifi at home so my data is always off and if my 100mb is finished oli top up with once off prepaid data.Please can this be rectifie immediately.