Duo Multi-Factor Authentication uses the devices, like phones or tablets, that you enroll to contact you for authentication.
If you are using Duo, you should already have enrolled a primary device, but it is important to have more than one device enrolled. Backup devices could be an office desk phone, a home landline, a tablet, or in special cases, a hardware token. Enrolling both your office and home phone numbers ensures that you can log in, even if your mobile device is unavailable.
Watch this quick video (0:58 seconds) on how to enroll an additional device in Duo.