Wired or Hard-Line Equipment
I'll start with some "Wired" equipment, since most people can relate to having wire between two or more locations. I'm not a "Telephone Guy", even though I did work for Western Electric out of college. I'd always wanted to work for AT&T Long Lines, but that's another story!
The military is (or was) fond of using "Field Telephones" for semi-permanent locations. Popular US equipment were the EE-8, TA-43, TA-312, and the later TA-838 units. Like most US military equipment, it was robust, well-designed, and easy to install and repair. If you can get these cheap (which I doubt!), then go ahead and use them. These units will require a battery to power them up, and some good old 'twisted pair' wire to connect them. Some of them even had a Ringing Generator to alert the person on the other end you were trying to call them. These were hand-cranked magnetos, and could put out one heck of a jolt if you got across the line while it was being cranked! Like most phone systems, the "Talk" circuit is D.C., and the "Ring" circuit uses A.C. There's nothing special about the wire used to connect the phones. As long as it has two conductors, and can withstand the environment, you should be good to go. Lamp cord (or zip cord), "Bell Wire" for doorbells, Thermostat Wire, or speaker wire can all be used. I've even seen people use electric fence wire since it's insulated from the posts by the stand-offs, and isn't *obviously* a telephone circuit. The gauge isn't that important, either. Anything 24 gauge or larger is fine. Stranded wire is preferred, as it unspools easier, and is more flexible. Telephones in the US generally use two colors for the wire that makes up a single "circuit" or "pair", red and green. The red wire goes to the "ring" connection, ("Red Right Ring" as they say), and the green goes to the tip connection. "Tip" and "Ring" come from the names of the parts of the plug the operator would push into the switchboard when they wanted to "connect" the calling party to the called party. They looked very similar to a modern stereo head phone plug. The 'tip' is the very end of the plug,, with the point on it, the 'ring' is the little band of metal further up the plug, and the body is called the 'sleeve'.
Since you don't want to pay "Collector's" prices for simple telephone equipment, two old desk phones can be wire up to make a perfectly serviceable comm-link. This guy has a good article on using an old desk phone as an extension to his working field phone, and there's tons of people with explanations of how to set up a simple phone system for very low cost.
Here's some Google hits to get you started with a wired system:
Farm Collector, Military Phones, Surplus Phones
And if you're interested in old phone stuff in general, try the Antique Telephone Webring.
The prc68 website also has a good list of military telephone stuff.
And if you're interested in the old Ma Bell stuff, here's the evolution of the American Dial Telephone.