This is an idea I had around x-mas because I wanted to know what I was getting so I started to mess with an old cord phone and this is what i got
Step 1: Things You Will Need
2.a flat screwdriver
3.pliers or wire cuters (something to cut the mic)
Step 2: Open the Phone
un-plug the hand set
open the hand set of the phone like showen in the pic below with the flat screwdriver.
PS.make sure not to break things if you want to put it back together.
Step 3: Unscrew
unscrew the screws holding the phone in place to get at the mic
Step 4: Gut It
once u have unscrewed it cut the mic and I would take all the buttons out of the phone because when you listen you don't want to accidentally push one
Step 5: Put It Back Together
you should put it back together and your done project should look like this