Bluetooth disconnect during a call : iOS7 on 5S

I have similar issue w my 5s and bmw x5 and noticed problem occurs when have phone connected through both usb interface and bluetooth. After disconnecting usb cable and pairing both 5s and car again from sckratch seems to work perfectly. Still testing it.

Based on suspicious proposed in other blogs, forums, etc. it seems that the bluetooth is prone to disconnecting if a call is received or placed while the iPhone is syncing CONTACTS to the cars system. I have over 500 contacts in my address book which can take some time to sync in the background. What I have done that has worked thus far is to create a GROUP in the contact book of my iPhone. I named it BMW and placed my most used contacts (as well as family and close friends) into this group. The group size is somewhere around 100 contacts. In bluetooth settings of my iPhone I have selected only this group to sync to my car. This syncs fairly quickly in the background I think and therefore reduces the risk that I am going to receive or take a call in the first few minutes I am in my car. So far I have not seen the dropped call issue since I started this workaround. This is DEFINITELY a problem either from BMW or from APPLE.