This article explains how Lifelines work. 

A Lifeline is a phone number that belongs to somebody you trust and that you can contact easily. This number will receive a verification code that must be shared with you to complete 2FA. Only phones can be used for Lifelines.

Note: A lifeline is not part of DUO and cannot be used to log in to normal university services through WebAuth. It can only be used to access the account management site where you can print temporary codes and where you can address device registration issues, which you should do as soon as is possible.