How to build a 3-way phone Step By Step: 1] You will need two different lines for this. Okay first take off both of the boxes covering the wires 2] Take the green and red from each box and attach a wire to each of these one wire to green 1 wire to red same on the other box 3] After you have 4 wires (2 For each box) then get a two way switch with two terminals then hook the two green wires to one side and the red wire to the other side then when you switch the switch you should here a dial tone and then just dial out and you will be able to talk to two people at one time [ Note from Tommy: When I was much younger and stupider I tried something like this... it worked for about 10 seconds at a time then the audio returned from the two far ends of the 3-way started to cancel each other out s l o w l y by phase inversion and conversation became impossible... if you're going to try this at least put a pre-fabricated FCC Part 68 interface on each physical line you connect... at least then your own incompetence won't draw unwanted attention from your local service provider, who I can assure you has significant clout with your local police... hint hint, take or leave as you wish... ]