When you're adding extensions to an entity (User, Group or Auto-attendant), sometimes that extension already exists. Since an extension is an alias for a specific entity this can cause an big issue.
If the entity already possesses the same extension, the system will alert you, giving you the option to either reassign the extension by choosing, "Yes" or cancel the change by choosing "No". (See Below)
If you reassign the extension, the original entity will no longer have an extension, and will not work. You will need to enter a new extension to the original entity in order to get it working again.