This is designed to work with the old-style handsets with the carbon mics inside (see picture). It probably won't work with most newer handsets.
I'm sure twelve year olds in 1994 used to call this a "purple polk-a-dot box." I apologize for my |33t h@><0r ignorance.
Step 1: Go get stuff.
1. Terminal-Block Cover (Radioshack Part #279-455)
2. AA battery holder (Radioshack Part #270-401A)
3. Micromini Toggle Switch (Radioshack Part #275-624)
4. 1/8" Mono Jack (Radioshack Part #274-251)
5. 1 AA battery
6. 2-5/8" x 2-1/4" cover (aluminum, plastic, cardboard, etc...)
7. 3/16 x 1" nuts and bolts (or one size shorter)
1. 1 roll black gaffers tape
2. 1 roll white gaffers tape
1. Soldering iron
2. Hot glue gun
3. Drill (3/16 and 15/16 drill bits)
4. Flathead screwdriver
5. Needle nose pliers
6. Wire stripper
Step 2: Prepare the case.
Once the plastic tabs are removed, drill a 3/16" hole in the side of the case at a right angle from the phone jack. This will be for the toggle switch (see picture)
Next drill a 15/64" hole in the bottom of the case opposite the phone jack. This hole will be for the 1/8" mono plug (again, see picture).
Take the 2-5/8" x 2-1/4" piece of material. This will be the lid to your case. Drill a 3/16" hole in your lid approximately 1-1/4" inch in from each side (see picture). Measure your material for better measurements and better results.