If you set your Duo authentication through your mobile device (such as a smartphone or tablet), you are encouraged to add a backup telephone number to your Duo authentication options in case you forget your mobile device. A landline in your office can be a good choice; if you are adding a Google Voice number as your secondary telephone number, the process is the same as adding a landline.
Add a Landline Phone from the Duo Authentication Screen
From the Duo Authentication screen (Note: you will see this screen anytime you are prompted to verify your identity with Duo, i.e., following the UMN Sign-in screen. If you have enabled the "Remember me for 7 days" setting, you can click cancel to return to the Duo authentication screen.
Select Add a new device from the left menu in the Duo Authentication box.
Authenticate with Duo using on of your other devices to ensure that it's really you adding a device.
Choose landline from the list of device options
Enter your landline number and confirm it is correct.
The device you added will appear in your list of Duo-enrolled devices, and you may begin to use it to verify your identity.