The doctor's fax number, you mean?
EDIT -
Actually, you're right. If he said it out loud over the phone, anyone in the room (with a good ear, he may have said it quietly) could have penned it down and used it later I guess. But I think caller ID and voice are pretty strong credentials.
What really worries me is how easy it would be to "wiretap" a cell phone. That'd make an interesting blog, iago. How is the data transmitted, and it's it cyphered or not?