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!

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