Step 1: Take Your DECT Phone and Headset.
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
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
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
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...
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!
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!