If you still have access to the old phone:

Follow the instructions HERE to set up the new phone on your duo account… then remove the old phone.

If you Do Not have access to the old phone:

Contact IT to clear your Duo, then use either the web or mobile instructions HERE to set up the new phone.

Because Duo will need to send a push notification to set up the new device, you will need to contact IT for them to remove the old number. Once done, you can now add duo to the new phone using either the web or mobile setup instructions provided HERE..