You need an old cordless phone. (one that you dont care about.. because you might break it) try and get one that has an "intercom" because they are much easier..
IMPORTANT- unplug this from your phoneline.. you dont want your telephone company after you.. also you can get a nasty shock.. when the phone rings.. a 110v signal passes through your phone line..
Step 1: Open Her Up..
Step 2: Have a Look Around
If you have a phone that had an intercom- there will be a microphone and speaker wired into the cicuit board.. you can snip and strip these wires and go onto the next step.
If you didnt have an intercom.. youre going to have to guess and check.. i used a cd player to put sound signals into various places to find an input.. and listen on the cordless part.. (you have to press "phone" or "on" or whatever on the phone) then do the same thing with some battery powered speakers (or maybe headphones) to find the output. If something gets hot, then find a new spot.
Step 3: Sabotage Some Headphones.
Then solder the cords to the places that you found inputs and outputs. (or the wires to the speaker and mic if you had an intercom)
you may want to try this with crocodile clips first.. becasue i found that i could only attach 3 wires.. if i grounded it twice with the 4th.. my sound card got angry.. again- guess and check, sorry!
Step 4: Test It
You can use skype test call, or windows sound recorder if you're on a PC.
try adjusting your sound.. i turned WAV way down for better sound quality.
again you sound card might only want you to have 3 wires connected.. so try that out, and switch them around etc..
Step 5: Re-assembly
feed the new wires through any available hole, or drill a new one.
if it all works, your done!.