Comment by devkit1

I think WarOnPrivacy was thinking of the T.38 standard. Once T.38 gained enough adoption in the various providers fax over IP became far more reliable.

https://en.wikipedia.org/wiki/T.38

edit: That is I assume that they meant VoIP when they say digital phone lines. Fax over digital ISDN lines has been reliable since its inception as far as I am aware.