I've never run into this problem of not being able to cancel a pending charge for cell phone/direct tv/etc. If I want to, I just log into the account, and change/delete the credit card info.
I would have to know more about the specific charge that couldn't be cancelled to be able to tell more. Others may have had a different experience.
If this is a major concern, don't set up automatic billing. Just call/log in every month and pay the bill with you credit card and do not give permission for automatic billing.