Billing department

I'm sooo fed up with the incompetence of the mtn billing department. I did a early upgrade for me and my wife paying the device remaining charge in store of R810 for myself and another R810 for her I have the invoices and proof that I payed it they can see it on the system that I made the payment. 2 months later I get the same amount added to my bill saying I now owe them this amount all they need to do is allocate my payment to the account they are apparently unable to do this. This was in September last year it's now been 8 months and still nothing is being done regarding this I have been to the mtn store 2 times in a month every month from September I'm waiting for my contracts to run out then I'm going somewhere else I have been with mtn for longer than 15 years I have never seen such ***********s in my life I get phoned weakly from the mtn ports department telling my they are going to cut my line.. my number is *** I'm going to stop paying this account and go somewhere else unless they sort out this matter sooner rather than later