Debit order

I took a contract with MTN and I gave them my banking details and advised that I will pay via debit order. I never received any statements or anything form MTN and last month I received a call from someone saying that he works for the MTN lawyers and wants me to pay them for the contract I have with MTN. I advised the person to send me an email, which he never did. I’m confused as to why MTN never took a debit order from my bank. I also don’t understand why I never received a statement. I went to the lady who works for MTN at Mutual Park and she said that they will send me a statement and she never did. I’m confused and angry at the same time as I have now a big debt with MTN and it’s not my fault that they didn’t deduct money from my account.