If I understood their paper[1] correctly you can just use a popular phone number that a person walking by is likely to have in their contacts, like an emergency line number.

My mobile telco's SIM comes with a couple of contacts on it like their customer support line. I don't know anything about AirDrop, but I assume this will work.

[1]: https://www.usenix.org/conference/usenixsecurity21/presentat...