Short answer: "it depends".
With old 'Bell 103" type modems, 'copying' the line communication was comparatively trivial with nothing more than an off-the-shelf modem.
With 'Bell 202' type stuff, it was even simpler -- they worked that way automatically.
Bell 212 type stuff required some _very_ minor hardware tweaking to turn an off-the-shelf unit into a 'tap'.
Above 2400 baud, you need some "somewhat specialized" equipment. Nothing terribly exotic, but you have to know 'where to look'. Generally impractical to modify 'retail, off-the-shelf' modems to the purpose.