Same problem in my 2006 M5. It seems to cut out in the beginning of the call. The recipient of the call can hear me, but I can't hear them. Most of my calls go like this:
Me: Hello?
Me again: Hellooo!
Once more: HELLO!!!
Call recipient: Dude, what the **** are you yelling about, I can hear you already!
It also seems to get noticeably worse when the phone is in my pocket, or maybe I am just imagining that. This needs to be addressed.