Hi Behemoth
Customer services should pass you to retention (customer cancellation team) if you want to cancel! They should not cut you off!! If you really have no joy getting through then send a letter but make sure it's sent to the office detailed on the back of your bill and make sure it's sent RECORDED, otherwise Ntl may claim to have never recieved it, also remember it's 30 days notice as well from when they receive it before you will be switched off.
Welcome to the forum by the way