mtn robbing me

mtn is taking unrelated amounts out of my bank account. I have tried phoning them, I just get transfered from dept to dept ir the call is dropped. I have phoned 7 times. They say I owe them an amount from an old account which I never cancelled. They did not contact me or anything and now just taking random amounts out of my account without me giving g permission or signing anything. I am only ine ina. household of 3 who is a bread winner. because of this I can't even pay my rent this month. I am already a depression sufferer and this is about to push me over the edge.