[ghaff]

>I know next to nothing about faxing - how is the transmitted data secured? Is it?

Traditionally, transmitting a fax was just making a telephone call so it was vulnerable in the same way that any telephone call was to wiretapping. With fax services as exist today, you're basically emailing them a PDF and they're then sending that as a fax to someone's phone number. (Or the reverse.) So there's no real security other than that provided by the telephone network and the service's internal controls.

I had efax for a number of years but I believe they eliminated the free incoming fax number for free accounts and I haven't actually needed to send or receive an actual fax (as opposed to a scanned document) in years.