Good question. I would like to know the same. So far, the only way I found to do it is push the option on the phone itself that tells the phone to ignore the call. That kills the in-car ringing, but I would prefer a function on the car that could do the same.

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2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input