The important bit is the metadata plus the uniqueness of the phone number. Whereas an IP address or, as he notes, a SSN can be used by many people, how many people use the same cell number/phone? And the IP is tied to a specific location while the SSN has no real world significance except as a verifier. By contrast, it's easy to see if a phone number is live and where it is. What's better from a marketing perspective, especially since you're out in the world uniquely traveling around with it?

If that's the Google play - wow.