Cordless DECT phones are great, but mostly don't let you type two handed without breaking your neck! Headset ones cost upwards of £45, and don't even have a dial pad! So I took a spare and upgraded it. :-)
Step 1: Take Your DECT Phone and Headset.
http://www.best4systems.co.uk/product.asp?ProdID=6790&CtgID=1002 is the cheapest DECT headset I could find. It has no info at all, and it is still £45 before tax! Something had to be done!
First, get yourself a set of headphones. I managed to get this headset for £1 on clearance, but you can spend as much as you want. Next, get yourself your DECT phone, and open it up. Most designs make you take out the batteries before you can, and that's a good thing.
Try not to trash any clips that hold the phone together, too, since you want it to go back together afterwards.
Step 2: Prep the Headset
Strip the plugs off the end of the headset and strip the wires back a little, too. You need to recall which one is the headphones and which is the microphone, so you might need a label. My headset has different wires - 2 for the mic, and three for the ears.
I cut about 40cm of wire off, too, as it was too long. I want to wear this on my belt or back pocket without getting tangled.
Determine which two wires feed the two earphones, and which is the ground. Twist the two feeds together. If you are using a mono headset, this doesn't apply!
Step 3: Warm Up the Soldering Iron
Grab your soldering iron and desolderer if you have one.
Remove the wires for the ear and pop off the mic.
Now solder your headset wires to the remaining solder points, ensuring they are the right way around. Polarity shouldn't make any difference, I mean make sure the mic is to the mic points, and the ear is to the ear points!
Step 4: Add Some Strain Relief and Holes for Wires to Exit
Grab a drill and a set of bits. Find a bit that is the same size as your wires, and drill a hol for the wire to slip into. I needed one at the bottom edge of the board to get the mic wire to the other side for strain resistance when it gets pulled, as it was a friction fit.
I originally threaded the wire through the screen, by removing the screen cover and clipping a small part out of the changeable covers. This was far from perfect though, so I decided to add a hole on the side and replace the LCD protection.
To make a hole in the side, I simply took a larger bit that would take both wires, and drilled through the edge of the case when the halfs were pushed tightly together. Alas, you can see the mistake! When I put the board back in, I had drilled the hole in the only place that there was a discreet component in the way! So I drilled another one a bit further up...
Step 5: Testing... 1... 2...3...
Now put the batteries back in, and apply the power. Your phone should find the base station and light up like normal, and you should hear a beep/dial tone. Call someone and test it.
If you can't hear anything, you haven't soldered the earpiece wires on properly, and if they can't hear you, it's the mic. Obviously.
Step 6: Screw Everything Back Together!
This was the hardest part. For me, anyway. Even once everything weas tested, squeezing the case together so the wires stayed right where I wnated them was tricky. Once it was done, I realised the next day that it wouldn't recharge, as the mic wire was trapped between the contacts for the cradle charger. This was soon fixed, however, and it now works perfectly. I can work outside or in the garage and take calls without any trouble.
The handset can be dialled out on still, and is fully functional. There is even a speaker which still works, and one menu option is for "autoanswer" and another is for "keylock", which makes it about perfect. It also acts an an intercom to any other DECT handsets on the same base station!