on
Tells the controller the field's number changed.
A verified state is only meaningful for the exact number that was verified, so any change to the E.164 value resets the flow to PhoneVerificationState.Idle. Without this, a user could verify one number, edit a digit, and submit a form that still claims to be verified.
Idempotent: passing the same number repeatedly (as happens on every recomposition-driven callback) does nothing.