MBD Forcing me into debit order

I've been contacted by MBD almost a year ago. Cell C have handed me over, on my account. I needed to settle this account. I've been contacted 3 times in the last 4 days. Where they want to run this as a debit order and try to force me to do so. I've asked them I'd rather pay this cash. I don't feel comfortable with them being in my bank account. The person I've spoken to then told me a debit order makes them feel secure so then they would not phone me again. I've then asked, but why phone me if I'm paying the account. Because they are charging per phone call they make. And every month I'm getting a phone call. So I'm paying off there phone calls and not the actual account. I've already received 3 calls in the last 4 days. I've asked but why phone me if I'm paying. If I don't pay then you can mos phone me. So MBD is forcing me into a debit order running on my account. But I'm paying as promised. How can they force me into doing something I'm not comfortable with. And if I don't pay there phone bill every month and my account stands still?