When you get a new phone, you normally have the same number, but you need to reactivate the Duo Mobile app so that you can receive a Push notification.
First, download and install the Duo Mobile app on your new phone.
Then go to this web page:
Click on the link on the left that says My Settings and Devices. It’ll ask you to authenticate, so have Duo give you a call. Once authenticated, you’ll see your various devices.
Click on Device Options next to your new iPhone or Android. You should see an option to Reactivate Duo Mobile – click that link, scan the QR Code with the camera on your new phone, and you should be all set.