Although in practice I don't think it will be an issue, in theory issuing a chargeback on your credit card does not release you from any financial obligations you agreed to with a contract. And if that contract specifies that you must "call to cancel" I don't think "I emailed" will hold up in court (but IANAL). Of course with this FCC ruling that could very well not be the case, but in any case always be wary of issuing a chargeback and thinking the matter settled if you did actually have legitimate commerce with the business in question.