Dear Lazyweb, here is a wiring diagram, obtained at great personal dissatisfaction.

See, I wanted to use my beloved Shure i2c headphones as a headset with my computer. Here in this modern world, you'd think that shit would be standardized by now, but no. It turns out that the Shure model I have, with the microphone and 2.5mm plug, is not a "cell phone" model, but actually a "Treo" model.

So, when I went and bought a USB dingus and an adapter cable to go with it, I found that the adapter cable was wired wrong (only one channel and no mic.) Apparently the Treo wiring plan is... creative? (This also explains why the retro handset doesn't work right with the Treo: it's wired in the same "wrong" way as the above cable.)

So, I made my own cable. Which was approximately 500% harder than it should have been because, bizarrely, it turns out that 4-conductor 2.5mm jacks are near-unobtanium -- and 4-conductor 2.5mm jacks in cable rather than board-mount form factor are complete unobtainium. (Radio Schlock has 3.5mm 3-conductor male cable plugs, so at least the other side is easy to get.)

So, to do this, you need to buy one of the rare board-mount sockets (Digikey CP1-42515RS-ND) and swaddle it in electrical tape.

The money shot:

(G=ground, L=left, R=right, M=microphone)