Mtn stole my money!

I have had a contract with Mtn since 1995 and am actually listed on their system as a “VIP” customer. At the end of November my bank account was debited with R13,000 (my debit order to Mtn is usually around R1,800 pm). Mtn promised to reverse the debit order, but instead just credited my Mtn account! I complained and again I was promised that the debit order would be reversed. I am still waiting!!!! Who can afford to have R13,000 ******, especially at this time of year!!