the cable that connects to the phone jack? My father made the mistake of stripping it in order to connect it directly to the line
Those older phones used “crimp” connectors on the ends of the wires of the cable that connects to the phone jack. See below picture, similar to the one at bottom left and/or bottom right, and made for use on a #22 AWG wire. They can be purchased at an “electronics” store, maybe Radio Shack, and you can use a pair of pliers to “crimp” them onto the ends of the wires.