Verify users with phone number
Instead of checking captchas and verifying an e-mail address, it might be more convenient to users if we let a user verify his/her identity with either a phone call or SMS.
Instead of receiving an SMS or a call, another option would be that users call a number and we just verify the caller ID (no calling costs apply). We could route that through our Asterisk server and call some APi endpoint at keycloak.
Still we need an e-mail address for notifications at fairchat, gitlab or faircloud. And we once decided, that the e-mail is the unique ID. And we should really check, if it was entered correctly.