By Bill Reeve (firstname.lastname@example.org)
Adapted for instructables by Mouse (email@example.com)
Disclaimer: The procedure described here may not work for you - that's a risk you have to take. If it doesn't work, or if you break something, it's not my fault - it's part of the adventure's risk. I assume you know how to solder. If you don't, please learn before attempting this procedure.
Introduction: Hooking up an old telephone handset to work with a cell phone is simply a matter of connecting together the correct wires. The goal is to substitute the microphone and speaker in the old telephone's handset for the microphone and speaker in the cell phone's handsfree headset. We will do this by attaching the handsfree headset plug (the metal end that plugs into the cell phone) to the end of the coiled cord attached the old handset. The trick is to identify, and connect together, the correct wires.
Step 1: Materials
You will need:
1. the handset from an old, broken and late-model telephone (please don't destroy an antique rotary phone),
2. the coiled telephone cord that connected the handset to the body of the old phone, and
3. a wired handsfree headset that works with your cell phone.
You will also need a soldering iron and some shrink sleeving.
If your handsfree headset is significantly different from the one used in this example, you might need a method of measuring electrical continuity, like a digital volt meter (DVM) that can measure electrical resistance. If you don't own, or can't borrow and DVM, you can still make this work with a different headset, but you will need to identify the matching wires by inspection or trial-and error.