FWIW, Digits exposes your phone number to the app: > A successful Digits account creation will return a stable userID to attach or match with your user record,

FWIW, Digits exposes your phone number to the app: > A successful Digits account creation will return a stable userID to attach or match with your user record, and an oAuth token for the device. A verified phone number is also returned for convenience, but the user’s number may change at any time and should not be used for authentication. Also, I understand this might be an edge case, but I don't always have the ability to receive SMS. This can happen in places without cell service (but with WiFi). More importantly, it can happen when traveling abroad and using a data-only SIM, which is especially bad if you get asked to re-login due to the location change.