I think we need more details. In particular, what sort of
modem are you talking about?
Bottom line, though, is that if you can get the pin-outs for the microphone cable, you can probably make something that will
work for what you need. But you probably won't find an adapter that just plugs in--you will have to make it yourself, or have it custom made for you (which will probably be fairly expensive).
Good luck.