Christoffer Järnåker has done a very good hack to convert his old indoor wireless phone to a Internet phone that can connect to computer and use with Skype. What he did is soldering the 3.5mm plugs to the circuit board. The most amazing part is he is using the trial and error way to find out the output and input point:
So, how did I do it? Not that simple at first, but very much simpler at the end. I started with opening the base station (Duh!) to see what chipsets were in it. I was hoping for like an RF part a couple of lines on the circuit board and then a telephone circuit. Tji fick jag! No of course not. As any standard electronics they make use of a whole bunch of circuits, so I started looking up on the internet what they did and again – nothing to be found. Then I decided to make use of the highly scientific method Trial and Error and hoping for a bit of good luck.
By having the phone off hook I carefully inserted my little screwdriver here and there until a heard a click in the handset. And when I found that click I injected a signal from my MP3 player – and it worked! Then reverse, by blowing in the handsets microphone I used the regular headphone for my MP3 player to see where I could ‘hear’ the phone. By grounding on wire and using the other as a probe I quickly found the spot next to the first spot.
Nevertheless, I really like this hack. This really save a lot of money but requires a certain level of soldering skill. If you have a old wireless phone around and bold enough try it out!