Add and verify phone number for existing email user

[MekkyUA]

I've Keycloak (Quarkus) up and running on production with thousands of users registered via email.
The email is being verified and used as username.
All users can login via email address.

My question is:

How can I use this SPI or a similar one to trigger a screen for the user to add and verify his phone number as a secondary identifier or without touching his email address?