The phone numbers could be the public identities included in the rating signatures, (ex: "555-12345, 5 stars, <$10 signed by paypal>" signed by me.) Drivers ar

The phone numbers could be the public identities included in the rating signatures, (ex: "555-12345, 5 stars, <$10 signed by paypal>" signed by me.) Drivers are incentivised to stick to one phone number, and to publish these ratings (through p2p). The clients could sort them by recency, so the meaning of old signatures would eventually expire. Just one example of how it could work.