Cellphone Contract

I have a contract with them, I changed my banking details because it was closed as I was moving to another bank. I notify them more than 5 time since April 2018, even my complaint is not resolved, they keep on debiting on closed account and I am being charged the rejection fee each and every month. and this is affecting my credit record as Telkom even told me that my account is in arrears.. I phoned them several times using my phone but the issue is not getting resolved they telling me that the manager will call me back but no one is coming back to me ...The latest complaint was done in August but still not resolved