I don't have a firm answer but a good practice might be to have a standard disclosure form where new patients (and existing patients) can explicitly decide if they want their information transmitted via facsimile or if messages can be left on their answering machine/voice mail.
Some people have similar forms that explicitly say you must notify the physician/office if certain modes of communication are unacceptable.
Even if a patient wants standard email, I wouldn't do it unless you have encrypted e-mail or use a patient portal. You must apply appropriate safe guards to whatever method you use.