Collections nightmare

I have 2 MTN numbers one is just a normal prepaid number the other is on a month to month contract. I pay R400 at the stores every month for the month to month contract. The stores allocate 2 of my payments amounting to R800 to the wrong number and now I’ve been trying to get MTN to transfer the R800 to the correct number for over 2 months now. All their collections team does is to keep reminding me I owe about R900 which I am not going to pay until my simple request is done. How a simple transfer of fees to an account belonging to the same person can be an issue taking more than 2 months is just so frustrating . I wouldn’t recommend taking any contract with them. I just need the correct bill so I can pay them and cancel the stupid contract.