Lavarock7 described the solutions.
Here is an expedient way to make a 2-wire phone cable: you can do surgery on a 4-wire cable to remove a small section of each of the 2 wires that make the outer pair on the connectors. That will result in a weak cord, so using Lavarock7's suggestion, of getting either a line splitter adapter or a pre-made 2-wire phone cord, is better. Looking through your existing cords may turn up a 2-wire phone cord. You could swap that one, if it is in use, with a 4-wire cord.