The AT&T ComKey 416 phone system was introduced in the mid-1970s, and was quite popular for about ten years. It had most of the features found in "1A2" phone systems, but did not require a central control unit (the big box on the wall officially known as the "KSU" or "Key Service Unit").
All of the circuitry needed to operate up to eight phones with two lines and one intercom path was built-into one primary (or "master") phone. If a second primary phone was connected, system capacity could be doubled. Satellite (or "slave") phones provided the same features as master phones, but did not have any control circuits. A wall-mounting kit was available separately, for satellite phones only. Master phones have power cords and require AC outlets. Although up to 16 phones can be used in a system, there are only 10 intercom "addresses."
ComKey 416 phones are connected together with standard 25-pair cables. The phones were notable for their modular construction, which made it easy to replace defective components. Phones were made in ivory only, but faceplates were available in different colors, and faux wood finishes. Notable features (for the time) include a recall button (for fresh dialtone without hanging up), privacy with release, voice-announce intercom calling, two intercom paths, line-selectable electronic ringing, Direct Station Selection intercom buttons, conferencing, and monitor speaker.
Master phones were shipped from the factory for use on lines one and two. In a system with two master phones, one could be modified to work on lines three and four.
Special model phones provided features such as hands-free speakerphone. Additional circuitry could provide a fifth line, and music-on-hold.
Standard button functions:
In addition to the popular Com Key 416 systems. AT&T also offered several larger models that used modified 1A2 KSUs and phones.
The Com Key 718, 734, 1434 and 2152 looked like common 1A2 phones but had mechanical and electronic complexities, including conferencing, multiple intercom paths, recall, privacy, message waiting, and handsfree answering of intercom calls.
Changing the lines on a ComKey 416 master (primary) phone
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.