If you can find them at a Army Surplus, the cheapest and easiest to use is a couple of the Army's field phones (TA-312). No other equipment is needed. You have one at each end with 2 D cell batteries in each. The handset is "push to talk". And to call the other station, you turn the crank on the side, which sends ringing voltage down the line.

Here's what they look like:
https://www.myinsulators.com/commokid/telephones/ta43pt_9.jpg