Incorrect Billing Amount - not as per contract

On the 3rd of October I upgraded my phone and re-signed a new contract at Chilli Lane Sunninghill MTN. <br> <br> The agreed amount, as per my contract summary is R349 a month for 24 months Anytime 200 (300mb +LTE). However every month since then I have been charged R359 a month. I did call MTN in December to query this and I was told it would be corrected. It is now February and still my monthly amount is R359 a month instead of R349.<br> Please adjust my monthly invoicing accordingly and credit me for the previous months which I have over paid.<br>