Over billing

Good day<br> <br> Since my last upgrade 21/12/2014 I have had problems with my account. I have 2 contracts with mtn 1 on a my choice 100 topup and 2 on a ipad which in total must come to 450 p m. Both of them are on a top up which means I can't go over a limit. In my fist month they send me a bill of over 800 and the next month it was over 2000 when I tried to find out why they said I was on a open plan and I immediately emailed the guy that handle my contract twice! he never came back to me so I went back to the store to the guy i mailed twice and he said that I was on both my contracts on topup accounts and that he wil send a query regarding the problem. Now for the pass few months I refuse to pay till the problem is solved and after 4 month still no answer so in total 6 months and they send me a bill for over 4000 and according to my calculations I owe them over 2000. Past month I tried to phone their help line 808 to speak to a consultant but every time you phone you get the message that the call centre is close! This is really bad service from mtn and why must I pay if they don't deliver a good service? The customer is always right no matter what.