Authenticating using the Duo app on your smart phone is the easiest way to authenticate; however, there are other methods by which you can authenticate, you may:
Register a landline - this can be your home phone, a family member’s phone, or the phone in your hotel room when travel reopens. There is no limit to how many landlines can be registered for Duo to call to authenticate.
Register your tablet (iPad or Android) to authenticate and Duo can utilize the pass code or send a push option without having to use a mobile number.