Incorrect billing

I terminated one of the lines in August, [#***] I asked for thier termination cost and lady by name Mpho Phatswane confirmed the amount I needed to pay and I paid, she offered her email too for the proof of payment and I copied MTN too. September the debit went off well and I noticed that they did not debit me for the cancelled line. Now October they debited me R15 974.08 iv been with mtn for over 8years and never missed even one debit order. Call centre ignored my fury and kept saying it's termination fee [#***]. This phone was a lousy R220 per month. This is ruining my credit score I don't owe 15k mtn is full of nonsence. I still have 2 other accounts with them don't you dare mess up my other accounts because I will leave. How dare you guys?!!! Disgusted