Validation. Would you want 2FA / MFA as an API?
So I would build a service and expose an API for that. So your webapp sends a request to a my API e.g. "'verify email' : 'john.smith@examp.le'". My service handles all the logic needed and after email access has been verified your endpoint receives a response. Included methods: email, password, mobile.
You mean like this https://www.twilio.com/authy ?
Mainly, yes. Thank you for pointing me authy. The question still remains, who would use it. And what would be the volume...