My initial design has no voltage protection (the thing that you were building up until now).

So, realizing that this lack of voltage regulation is a problem, I built the circuit pictured below to kick down the output voltage from 1.5 to 1.2 volts (because rumor had it that this was line level). Line level is still a bit myserious to me, but thanks to a comment posted on the main page by Phatso I can now tell you that line level is probably more around 1v. However, the circuit below should still work fine.

Put together the circuit shown below and shove it inside your case (so that your mic works right and doesn't harm any other devices that you plug it into).

You can also try just using a 100K potentiometer and a 10uF electrolytic capacitor between the telephone jack and the mono jack. This, in theory, will provide volume control and protection from bad voltages. However, I'm not really sure how or even if this will work. This is mere speculation on my part (based on what someone told me once in passing).

Also, a low voltage LED to indicate whether the device is on or off may be nice.