Telkom invoice dispute and cancellation of contract

Telkom is billing me for a contract that I called and cancelled within 7 days. As per their contractual stipulation if you call within the cooling off period, you are not charged. The device was collected by the courier service two weeks later despite calling them several times. I now have a statement saying I owe money for a device that is no longer in my possession and Telkom refuses to cancel my contract despite me calling every single day and providing proof of the returned device.

Please help