In the old days, 4 wires came into the home and each colour wire was connected to each jack. The inner 2 wires (red, green) were LINE1 and the outer 2 (black, yellow) were LINE2. Each connected phone became another parallel connection across either red/green or black/yellow. Most homes just used red/green, but the 4 wires from the phone company support 2 separate lines.
With VOIP, the 4 wires from the phone company should be disconnected from the street at the electrical panel, but I think most people don’t bother checking this. Instead, they just plug in their adaptor to a phone jack, which turns the adapter into the equivalent of what previously came in from the street (2 pair).
The OP needs to isolate the basement LINE1/LINE2 wires from the rest of the house, at which point there are lots of options for giving the tenant a separate line, but this is what I would look at doing first.
Cisco 8642 500Gb PVR