Or you can just read it directly. Just need some light.
It's actually better than plain text stored on a Hard Drive/ CD/ Floppy et c., which requires corresponding reading devices, format parsing systems, a display to show it and an appropriate power source, after which you can consider using a human to use the data (or remove the monitor and convert data into other data, in which case, you need another output device/network).
In principle none of that data should leave the phone line. Dunno whether carriers encrypt VoIP but in any case it shouldn't leak into the internet. Back in the days it was considered secure because in practice it's indeed similarly secure as a letter: In organisational terms, yes, in computer science terms, hell no.