The intelligence community has been intercepting faxes for decades.[1] It's not very secure. Even worse, VoIP systems recognize fax modem signals, decode them, transmit them as data, and re-encode them as fax modem signals.[2] They have to; VoIP has such awful jitter and bandwidth that you can't send modem signals over it. So it's easy to peel off the fax data and log it. This is done routinely using fax servers.

[1] https://en.wikipedia.org/wiki/Black_site#The_Onyx-intercepte...

[2] https://www.dialogic.com/-/media/products/docs/whitepapers/1...