Ext search in "Add User" creation wizard

When adding a user in RingCentral MVP, finding an available extension can be time-consuming.

It would save time if the "Ext." field displayed unused extensions when you click on it. Currently, you either type in an extension and hit "Next" to see if the extension is in use or you have to go to Users > Users with Extensions and scroll through the user accounts looking for gaps in assigned extensions.

Having the ability to search for extensions that are not in use would be helpful in the case that you need to narrow down the extension search. For example, typing "3" in the field should filter extensions that begin with "3".

Attached is a mockup of what I imagine this feature may look like.