If what you say is true. Then the system is currently equipped for 8 digital phones, 4 co lines, 4 single line extensions. Therefore if you had 6 originally and want to add 2 more, you should have the available 2 spots to do it. Looking at the processor card, there are p1-p4 those are 4 digital extensions there. Then look at the Uni-Card, to the right of it are 3 lights. Whichever light is red, look at the p number. Then look at the card, the P1-P3 are all setup in groups of four. 1,2 on top 3,4 under 1,2 for each card on the unicard. If you have 7 things plugged into the digital stations, then you have a jack somewhere that is active and not being used with a phone. I would test by dialing the extension numbers. If only 201-206 work, then you have only 6 in use. Dial 207 and 208 to see if they are active. Be aware that they might have skipped a port or 2 in installation. Just try calling each extension from 201 and see which ones are not in use. More than likely your system will be setup as 201-204 on the processor card and 205-208 on the uni-card, the first 4 ports going from top to bottom.