Bad service and billing system

My grandmother took a contract out for a P10 lite at the cost of R269 a month August 9th 2017 we have been having a billing issue with them and since last year and every month the bill ranges between R500-R1500 and they can't explain why our bill is so high, we phoning into the call center and they can never help us always transferring us to another person, we will not pay this bill for a ridiculous amount until they fix it