Cancellation of gym contract

I am struggling to cancel my gym contract for almost a year! I've tried numerous times to get a manager to phone me back and to refund me on the remaining term of my contract. When I try to phone the gym, the number does not exist. I am really getting frustrated about the service I receive and that NO one has phoned me back to resolve this issue. In the contract it is stated that the contract can be cancelled with a 20 days written notice, which I have done twice!