Note - You will need basic electronic skills to build the project in this tutorial. You will also need to do some research to verify whether or not it will work with your phone and what connectors you will need. Use this information at your own risk; do not complain if it doesn't work for you. Only use as prescribed. Your mileage will certainly vary.
Let me state it again since some folks have ignored it in the comments. I am NOT providing a specific schematic, details on the connector for any particular phone, or more details on how to hook this up. It is an extremely simple circuit if you've got the right skills and knowledge and the ability to use a search engine to look up the details for your particular phone and verify that it'll work with whatever handset you can get your hands on. DO NOT ASK!
Here’s an interesting retrohack which is actually more practical than it seems at first glance – connecting an old telephone handset to your cell phone.
It’s been done before, primarily with Bluetooth. Many of the mods just repackage a Bluetooth headset by removing the handset’s original microphone and speaker and putting the Bluetooth mike and speaker into their locations with whatever extra wiring is required. A few use the original handset’s mike and speaker and connect the wires to a Bluetooth module. But this version is non-invasive (doesn’t change the handset) and is corded.
Step 1: Why a corded handset
Bluetooth and cell phones are a marvel of miniaturization. Remember what cell phones looked like a decade ago? They were almost as large as the communicators from the original “Star Trek” series (which ironically was set in the 23rd century.) Miniaturization results in compromises. The current technology for tiny speakers and microphones is pretty amazing, but the quality sometimes leaves a lot to be desired. When I use my cell phone at home I usually hook it up to a repurposed pair of computer speakers to make it easier to understand conversations. So using an old corded handset with your cell phone may result in better voice quality.
In addition Bell Labs put a lot of research into design ergonomics for telephones. They’re designed to be comfortable (within the technology limitations of the time and given that they had to be produced for a reasonable price). Some people actually prefer using a standard phone handset instead of holding a cell phone up to the ear, having a Bluetooth unit stuck inside the ear, or wearing a headset. Unfortunately ergonomics design seems to have been forgotten in today’s cell phones. (What in the heck was Samsung thinking when they placed the far too low volume speaker on the back of my phone facing away from my ear?)
The specific description in this tutorial is for a phone with a four conductor (TRRS) 3.5 mm. mini jack which is used for stereo audio output and mono mike input, commonly found on many iPhones and Androids. If you want to build this adapter it’s your responsibility to make sure that this is the correct connector for your phone, or what plug is required and its pinouts and wiring diagram for your particular phone.
For example, here’s how somebody wired an older cell phone which uses a 2.5 mm (micro mini plug) with three conductors instead of four.
In addition telephone handsets have evolved over time. The original carbon microphones have been replaced with electret mikes. In some cases telephone handsets are not interchangable because the manufacturer used a special microphone in the handset and the circuit in the phone which encodes the voice is designed to work with that microphone.