Mislead by consultant on payment

I Called in to Mtn every month to get my payment due for the end of each month. In january 2016 I called in to mtn to find out what's my payment due for end of January and I was told by the agent my payment due is R685 on a recorded line. However I made a payment of R600 at the end of January and my line was barred in February. I then called them back and they advised me that am in arrears of R85 and made an arrangement to have the payment made at the end of February. So I called in February to find out what's my payment due for the end of February and the agent that spoke to me told me that it's R1500 which does not make sense to because I have a top up cell phone contract which is R599 per month and a data contract R104pm that does not add up to R1500. The lady told me am in arrears and I must wait for the investigation to be done. This is unfair as a client and they said I have to make the payment.