How to create custom passwordless SSO login?

We’re developing multiple web apps (ie: & We want our users to login “passwordless” using their country ID, and OTP code sent my SMS, to any of those domains in SSO fashion. (using auth0 universal login, the only way possible is letting the user enter directly his phone number, but we want him to enter his ID, not his phone number directly)
We have a backend service holding the users ID & phone number relation
(ID is just a user field, it can be different one, what’s important is that in the backend we can tell the phone number by that unique field)

We need a seamless SSO solution for and If a user logs in on and then goes to, he should be seamless authenticated without re-login. Otherwise, he will be redirected to the custom SSO login.

What is the best way to accomplish that?

Is it possible to modify Auth0’s universal login to support ID (or any other unique field) based SMS login?