I just looked up Apple's Security White Paper and it explains that the Bluetooth LE signal only includes a "short identity hash", and then the receiving device

I just looked up Apple's Security White Paper and it explains that the Bluetooth LE signal only includes a "short identity hash", and then the receiving device responds with its own identity info, and then the sending device establishes a peer-to-peer WiFi connection and sends its "long identity hash", which the receiver then uses to confirm that it knows the recipient before responding with its own "long identity hash". Which it to say, it sounds like you only get partial information about the sender prior to identifying yourself and establishing a direct connection. So you cannot passively identify the exact phone number of an unknown sender. The "short identity hash" itself it says is "created based on the email addresses and phone numbers associated with the user’s Apple ID", though I have to assume that it's independently hashing each piece of info because if you hash the whole set, then the receiver would have to know the whole set, which would be a problem (my device knows more email addresses for me than any of my friends do).