For the US Army, there were two kinds: a light insulated "Assault wire" and heavier gauge "Field Wire". A mile of Assault Wire was 30 pounds, a mile of Field Wire 130 pounds. Here's what some old Field Wire looked like (the photo on the right), twisted pair of wires with rubber insulation, and a braided cloth cover over that. Each had 4 steel strands with 3 copper ones. That steel was half-hard , almost spring wire. As a result, the cable was quite strong. Not enough for towing vehicles, but able to take being driven over by one. When spliced, instead of soldering it was given a Western Union Splice and then wrapped with the typical tar-soaked cloth electrician's tape.

Here's a useful site on the Signal Corps, and here is the Army Technical Manual for the phone.

Eventually the W-110 field wire was superseded in the 1950's by a lighter wire that was wound onto and off the spool in pairs, and that had a more durable harder plastic insulation, but that still had the 4 steel with 3 copper strands. You can still find and buy that pretty easily- by the meter or the mile- and the later field telephones from the 1950's too, if you want to play soldier. They take D-cell batteries!