MTN NOT ALLOCATING PAYMENT

I had 2 contracts with MTN, recently moved one to prepaid in December 2017. 29th of January I paid my MTN account via EFT but mistakenly used the now prepaid number as reference instead of using the current remaining contract number as reference. To date I have contacted MTN 8 times after they suspended the other number due to non payment. I have asked times without number if they can allocate the payment to the existing contract since they can see the payment done to a number that is now prepaid. No one is able to do this , at the end im now supposed to pay again because the credit department of such a big organisation prefers to have our funds in their suspense account and you need to pay again for their lack of knowledge and inability to process something as simple as this. Im disappointed in the lack of experience in such a matter. I even sent thru my bank statement as proof of payment , they refused.