Duo Device Management Portal says my phone number "cannot be added"

If the Duo Device Management Portal is showing this message:

"[phone number] cannot be added"

it means that you're phone number already exists in Duo. If you are trying to add a new phone and recently removed your phone from the Device Management Portal, it's possible that your previous phone was connected to another account. For example, perhaps in addition to your individual user account, you had your phone also connected to an administrative or shared account. The previous phone will likely need to be removed from all connected accounts. If you don't know what those accounts are, please contact the IMSS Help Desk for assistance at x3500 or 626-395-3500.