Pick an extension. Assign it Remote Call Forward (#322). At the phone, program the outside number into one of the personal speed dials (Feature 99 for instance). Add a Call Forward button to the phone (Feature 11-(Extension Number)-(Speed Dial Bin (99 for instance)). Press the button, it will turn on and you will see Forward XX 99 on the screen.
Be sure the extension being forwarded has lines assigned to it, and that the auto line select grabs lines first instead of Intercom first. Make sure the extension is not restricted from dialing the outside number (Press Feature 99, make sure the call goes through)
Now go to another extension, press Intercom and dial the extension number of the forwarded phone. You should see a line go off hook, and the outside number should ring.
Now call in, answer on an extension, press Transfer and the extension number of the forwarded extension, hang up. A line should go off hook, and the incoming and outgoing calls should connect.
Finally the important test - hang up one of the phones, make sure both lines disconnect.
If everything passes muster, program one of your selector codes to transfer to the forwarded extension, and keep it forwarded.