A DuckDuckGo search for "phone line splitter" (or "duplex phone jack", for a more "correct" term) comes up with a multitude of responses. They all look like this little beauty:
images courtesy of homedepot.com. No endorsement intended or implied. Click all images to embiggen.
The HD also sells triplex versions for one in, three out. I'm sure that, at some point in time, one could purchase 4- or 5-output versions of them, too, but fewer people need that many land-line jacks anymore, so they're becoming a bit of a dinosaur.
Internally, these are nothing more than incoming lines to a punchdown block that the outgoing lines are connected to. There are absolutely no smarts inside this thing at all. That's what allows you to pick up two handsets and have a "conference" call at your end.
The good news is that the duplex one (currently) lists for less than $3, so you don't have to pay expensive network switch prices.
If you need more lines, you simply daisy chain more devices.
The other option is to install a puchdown block like this one:
image courtesy of amazon.com. No endorsement intended or implied
You put your incoming line into the blocks on the left (using a punch down tool), then run one line from each of the blocks on the right to wherever you need them. This, plus the tool, are considerably more expensive than the little outlet duplexer above, but are probably more reliable.
For a dirt cheap, DIY solution:
- Drill 4 holes into a piece of wood.
- Epoxy a nut over each hole.
- Run a bolt and two flat washers through each nut.
- Strip your phone wires
- Clamp all the red wires between one pair of washers
- Repeat for the blue, yellow & green wires, each on their own nut/washer/bolt set
You've made your own "punchdown" block. This is, literally, all there is to it.