Unfortunately, there is no way to 'cancel' the fax once it has been sent out. If you faxed it to the wrong organization or party, we recommend that you contact the recipient to inform them about the fax containing sensitive information. If the information is extremely sensitive, you may also wish to report the matter to your local law enforcement.
Otherwise, if you typed in the wrong fax number by a few digits, in most cases you may just see an error message saying 'Invalid fax number'. In this case, there is nothing to worry about.