I find cell phone use in an open car to be challenging at best, impossible at worst. My bluetooth headset is a dismal failure when talking with the top down at anything over about 15 mph. I would think a small microphone someplace in the windshield surround would be worse than that.

Someone turned me on to the Shure QSHB4 headset with a boom mic and a foam wind screen. Yeah, it looks a bit like an early Britney Spears on-stage unit but sound clarity is fantastic for both me and the party on the other end of the call. And that is with the top, and windows, down at 80 mph!! The thing really is a great piece.