change of banking details.

I have had a cellphone contract with Cell C for the past 7 years, i have changed my banking details and i have been trying to update my new banking details with Cell C for the past 4 months, i have spoke to 3 different consultants and they all confirmed that my banking details were successfully updated, however my Debit order still goes off from the old account. The issue is that i am paying extra bank charges, i have to do a cash payment/EFT transfer and they also charge me for an unpaid debit orders that are really not my fault. This is very frustrating as i have been calling and the agents keep on transferring my call with no assistance, apparently the supervisors and managers are not available to assist.