Air Marshall Eldritch

This is a repost from another forum...

Is there any way to piggyback my regular mobile handset voice channel to the existing hands-free setup in my '98 E? I don't mean the actual service, which has been covered here already, but just the sound.

I use a little cheapy earpiece with in-line microphone which plugs into my handset, but the car's mike and speakers have to 10x better.
Remember those old CD players that used some sort of fake cassette tape that went into the tape player? I'm thinking along those lines.