compact ics

I am trying to add an extension to a Compact ICS phone system. This system has
always had fewer than 9 phones. They have 7 handsets and voice mail hooked up to
port 8. They are trying to add a 8th phone to the system. When I cross connect a
phone to ports 9-12 I just get flashing arrows on the buttons display. Is this
phone system only equipped with 8 ports for extensions or is programming needed?
Please Help!