2nd possible reason for the error: Your google access token does not contain the required permissions(How to get the google access token is different from Auth0 access token, its stored in User object of Auth0, you will need to access it and check the google access token)
If the google IDP access token does not have the required permissions, people API will not return the phoneNumber also. Remember returned access token by app in auth0 is not the Google IDP access token its a Auth0 token You can either print the Google IDP access token in the rule to see the value (example above :console.log(google_access_token); ) or you can access it using the management API endpoint /api/v2/users/{user-id}, will be present in identities array, once you get it, you can verify the permissions using,(https://oauth2.googleapis.com/tokeninfo?id_token={token}).
Do debug these possible two reasons and let me know how you go!