[keycloak-user] SMS customize authentication workflow

Dear experts, after the successful login we would like the person to be challenged by a SMS sent on the phone. We think that the best option is to customize the OTP interface where, instead of trigger the procedure that will wait for the OTP challenge, will call a web service (already developed) and it will wait for the user entering the SMS received on the mobile. What steps should we perform to reach this goal? Thanks in advance