Telkom wants to withdraw money from my bank account ********ly

I want to cancel the expensive package I have with them to go on to a cheaper package. The call centre person , Shannon Morang , insisted I give her my bank account number. This is after I already gave her my ID number and confirmed addresses , etc. Why would they need my bank account details if I pay by EFT ??? Telkom is losing so many clients already, I should think they would want to hold on to the suckers that do still use their services ??? I am most definitely NOT supplying them with my bank account number. The next step logical will be to give up the line.