Cellphone contract

I opened a cellphone contract with Telkom at Kollonade, debit order went well for 3 months, thereafter, Telkom has been debiting my account 3 times in a month, with different amounts. I called them to enquire, they asked me to send an email together with my bank statements and promised to resolve this matter, i made follow ups, till now nothing is happening. I reversed and asked my bank to block the debit orders, the following month they change the code and debit my account again. I don't even know what to do. I am so frustrated because even if you call them, they will take you from pillar to post, and leave you hanging until you hang up. Please advise me on what steps to take