Debit but cancelled service

I called Tracker to cancel my contract. This was still within the 20 days period. I was assured that they would not bill for August but they did. This was also recorded on our telephonic conversation. I called them yesterday to have it reversed. The consultant was to call me and provide feedback as agreed, as they refused to reverse. BuT why refuse because I called on time and we agreed it would not be debited. They have telephone records as proof.