Refusing to agree to a payment arrangement

Telkom kept debiting my account incorrectly throughout the year. Despite attempts to rectify this I wasn't able to. Now they want all the outstanding money in one payment. It's a huge amount and I'm unable to. I've tried to make an arrangement with them but can't get hold of anybody that can help and they keep saying they will get back to me. They haven't yet. Meanwhile they keep taking money off my account and I keep reversing it.

They sent me an SMS where they say a 40% payment has to be made to even allow for a payment arrangement. And they will hand me over to external collectors. Well sorry, I'm trying to get a hold of them to discuss this. The fault was theirs in the first place for debiting incorrect amounts and not rectifying the issue. Now o have to suffer?

Please help me Telkom. I am unable to pay you the first 40%. I need to make a different payment arrangement with you, with IS ALLOWED BY lAW so please act within the law and get back to me so we can make this arrangement.

Ps I will NEVER use one of your services again. To this day you can't seem to get my address or name right either.