Here is the inside of the handset. I decided to take it apart to clean it as well as see how it looks inside. One thing I learned is that the white wires on the other end belongs to the speakers. I was hoping they'll be colored differently so I can tell the difference between positive and negative. I guess that won't happen...

There are 4 wires coming out of the chord, black, red, and two white cables. I'm guessing the black is microphone negative, and red is microphone positive. As above I cannot tell you what the white ones are, it may not even matter as it may just invert the wave form but still output where I can hear it.