A ComKey 416 "master" or "primary" phone comes equipped to handle lines one and two for all of the phones in the system. It's easy to convert it to work on lines three and four, or just three if you have only three lines.
(1) Unplug the power cord from the electrical outlet. Turn the phone upside-down and position it so the rear faces you. Loosen the two screws along the rear edge.
(2) Slide off the rear panel which holds the transformer and power cord.
(3) Locate the wide, flat plug on the left side of the phone. It should say "Lines 1 & 2."
(4) Carefully remove the plug from its socket.
(5) Rotate the plug horizontally so "Lines 3 & 4" are facing you. Carefullly insert the plug in the socket.
(6) Re-attach the back of the phone and tighten the two screws.
(7) Plug the power cord into the electrical outlet.
(8) Verify that all lines are working.