MTN HOLDING MY MONEY

I have made a payment of 9900 to mtn by mistake since I pay my premiums by eft, now I can see the money is reflecting on the mtn app. I have called, messaged, emailed and sent messages via Facebook asking for a refund. Mtn has been sending me reference numbers and saying I should send Proof of payment and bank statement the money came out of, I have done this so many times but MTN is not refunding me. It has been 14 days now!!