Most**US home telephone wiring is 2-pair, with https://en.wikipedia.org/wiki/RJ11#Pinouts RJ11 connectors. The primary line is on the red/green pair, and if you have a second line it'll be on the yellow/black pair. You can get two-line phones that use both, or two-line connector boxes that break out both lines into two jacks (using the red/green pair on both, so regular phones work.)
So if you've got one land-line, you can still build this intercom on the second pair without it interfering.
* *Re: "most" - Your mileage may vary, especially in older houses. Maybe they've got two-wire, and I've lived in a place that had 3-wire, and maybe the wires have old discolored cloth insulation instead of colored vinyl, so good luck telling which is which. But anything since the 60s or so should be useable.