When compared to analog faxing, is eFax secure?

Your question - Is eFax secure can be answered as - Fax over VoIP (which is normally what eFax means) is not any more secure than Fax over pure analog lines. To clarify, eFax isn't a technical term, it's a marketing term. The underlying Fax is still an inherently analog technology, and the VoIP technology underlying it is merely trying to emulate the analog lines. Normally the other end of the call is often going to be an analog line connected to the PSTN.

Furthermore, the VoIP portion of the call isn't encrypted, is sent over unencrypted RTP and hopefully T.38 and can pass through multiple carriers before arriving at the destination. There's no reason to think tapping these lines is any more secure than tapping an analog line. Wireshark has nice tools to intercept this traffic if you have a place to sniff. Without using some sort of proprietary technology on both the sender and receiver, it's not possible to encrypt Fax. If you're concerned about someone intercepting the transmission, the short answer is don't use Fax, use some form of digital communication that's encrypted.