Maybe I'm not explaining correctly, but in other BT-integrated cars I've owned/driven and in fact with headsets and other devices as well, they can "remember" which devices have been officially paired with them, and then whichever one is closest (or the most recently used, if multiples are present) is the "connected" device.
I've only got two phones paired with my x920BT, I don't expect it to auto-PAIR with my friends phones or the phones of people walking by on the street, but I damn well expect it to CONNECT with phone #2 when phone #1 isn't present, and vice versa. Like every other BT