You've created some user extensions, your phone is plugged in, but you still see "Unassigned" on the screen. What's next?
Click on Business Phone - Device Inventory
The phone should reboot in the next minute or so, initialize and you will now see your user's name and extension number on the screen.
You can assign a user to multiple phones in your device inventory.
The result will simply be that those phones will all ring simultaneously when a call is directed to that extension. Outbound calls will display the caller ID and name that was configured in the user settings.