VOIP: Cannot assign user to extension

[japrogramer]

When I assign a user an extension that exists in Asterisk things work well.
When I remove the extension in Asterisk, that user is no longer available as
a user to assign to a new extension.

The behavior should be so that each time an extension is removed from Asterisk it is also
cleared from any user who has been assigned the extension
Or
So that each time the user are checked for an extension it checks if the extension is valid and if it
isn't valid the extension cleared for the users with invalid extensions.

Using latest release