I wanted to do something a bit different one day at work (I had been working like a crazy man and had to give my brain something completely different to do to keep my sanity), so I decided that I wanted to be able to use a computer headset instead of the handset at work. The best part of this switch is that you don’t need any active or passive components to change impedance or signals!!
4P4C (sometimes called RJ9, RJ9, RJ10 or RJ22) plug information
To begin with, you must understand how telephone handsets are wired to the 4P4C plug. Fortunately, the connections are dead simple to understand. The 4P4C connector has 4 contacts. Contact 1 and 4 (the two outermost) are the microphone contacts (polarity doesn’t matter here) and contact 2 and 3 (the ones in the middle) are the speaker contacts. See diagram 1 for more details.
Diagram 1 - 4P4C (RJ9 RJ10 RJ22) contact diagram - ©DnetSvg CC-BY-2.5.
Schematic Diagram
Telephone headset switch schematic diagram
As you can see, the schematic diagram is very simple. The most complex part in this schematic is connecting the ground signal together 
.
Part list
2x 3.5mm stereo jack connectors, preferrably the screwed type.
2x 4P4C (RJ9, RJ10, RJ22) connectors
1x dual state switch
1x hobby box (plastic box to keep everything tidy)
Some spare wire (I used a bit of Cat5 cable)
Solder
(Super)glue
Assembly
Here you can see the internal wiring of the switch box. To hold the 4P4C connector, I used superglue.
Handset switch wiring
Below you can see the box completed. The only thing left to do is to label it and put it to use.
Completed switch box
The switchbox on my desk. It is really easy to use and the sound quality doesn’t suffer a bit (the sound quality in phone systems are horrible to start with .. )
Switchbox in use on my desk.
If you build this box, please let me know in the comments below!! 
My name is Sigurður and welcome to my website!
Hello Sigurður,
Great and clear post.
I’ve just made my own two days ago. I wonder if you get the problem of having audio only on one side of the headset, as I have.
If so, do you think it could be fixed by adding some resistors in the mono to stereo wiring before the 3.5mm jack of the stereo?
I’m trying (unsuccessfully) to find a schematic of that wiring (as this should be kind of common case), and I don’t posses enough electronics skills to figure it out by myself.
Regards!
Hi Salvador and thanks for the comment!
Do you have a stereo jack for the mic and the headphones?
I used stereo jacks and just soldered the left and right channels together. I get perfect audio to both of the speakers.
I didn’t worry much about matching the impedance of the speakers or the microphone since I get a decent power transfer. If you want to match the impedance of the speakers, connect the left and right channels together and add a ~10 ohm resistor in series with the speakers. You shouldn’t have to worry about the microphone (if your headset doesn’t have dual mics).
If you want, I can create a new drawing for you, but as a first attempt, solder the left and right channels together (shown on the drawing), and see if doesn’t work for you
Kindest regards,
Sigurdur
Great!
That’s what i want
Hi, great post, very similar to what I am trying to do on a linksys SPA942.
However, I am try to use a smartphone headset (an iPhone compatible one) which of course has a common ground between the audio and the microphone. Pin outs I am using are here:
http://pinouts.ru/HeadsetsHeadphones/iphone_headphone_pinout.shtml
In yours you have used RJ9 3 and 4 (green and yellow) as ground. So when I soldered mine, I soldered both 3 and 4 to pin 3 (ground).
When the phone is on mute, it works fine, but when I unmute it the audio immediately breaks up and then stops. The microphone also does not work. If I leave RJ9 pin 4 (yellow) unsoldered, the microphone does not work (which makes sense)
Can you tell me what I need to do about having two separate grounds?
Thanks
Kevin
Hi Kevin.
I don’t think you need to worry about two separate grounds, the phone is so new that the ground for the mic and the audio are probably the same.
Do you have a 4 pin jack, or only 3 pin jack? (You need the four pin jack)
/Sig
I found this:
http://www.freepatentsonline.com/20060234771.pdf
which seems to have a different contact configuration. He has joined 2 and 4 for a common ground. Tried that but no luck.
In fact I seem to be unable to get any microphone at all. Will keep trying.
I have a question, do you still have to pick up the receiver while on a call and then switch to the headset to use it?
Unfortunately, yes. I did not design any automatic receiver lifter with this (although, I thought about it…).
It’s not a big deal for me though.
Ah, I work for a small contracting company and have been looking into a way to use a standard 3.5mm headset instead of paying for plantronics headsets. Do you think it would be hard to incorporate a diy handset lifter?
I don’t think it’s hard to incorporate a diy handset lifter.
I would get a servo motor and create a two state PWM circuit, one for picking up and one for hanging up.
You would also need to create some mounting bracket to position the servo correctly for the handset.
It should be easy enough to design a PWM servo controller using a 555 timer circuit and a switch.
/Sig
Inspired by your idea, I have built one switch box for my phone between handset and headset. It worked very well. I have taken few shots of my box. But I don’t know how to upload them.
Hi Sean.
You can’t upload on my website, but you can upload the images on a public network like imageshack.us, picasa or flickr, then post a link to it here.
/Sig
Here is the link of my switch box:
http://www.flickr.com/photos/pier7/sets/72157631756807915/
Whoops .. seems like the images are restricted (I’m unable to look at them).
Sorry, my mistake. I have those pictures openned up for public now. Let me know if you still can not see them.
Sean, that’s such an awesome and compact build!! Thanks so much for sharing!!
/Sig