MFA - Email as a back up for SMS but not for OTP

Hello - for MFA - why is it that if you choose SMS as your authentication type, you have the option to use email if you don’t have your mobile phone on you BUT if you choose OTP as your authentication type, this is not an option?

We are using Auth0 MFA for our EMR and there are times where our users may not have their phone on them. Many companies require OTP over SMS and they are finding there is no back up if the user doesn’t have their phone on them.