The “Delete” button missing for one of my extensions?

Phone Numbers can be configured to either play the Main Greeting or directly forward your callers to an extension. When one of your phone numbers is configured to ring to an extension directly, that extension’s Delete button is hidden to prevent accidental deletion of the Extension. This would break your call routing. If you wish to delete an extension that currently has a phone number forwarding calls directly to it, you must first change the phone number to forward to a different extension or play the Main Greeting. Then the "Delete Extension" button will reappear in the "General Information" section.