That's the beauty of the older systems. You could rig up tricky stuff for unique applications. No need to consult with software engineers halfway around the world.

Sounds to me like the can relays were just 24 volt ones wired through independent circuits associated with the exclusion keys and buttons. No tip and ring with relation to the operation of the coil. This setup would require external power though, to operate the relays. Did they maintain a small bank of dry cells (that's what they called batteries back in those days) for this purpose during a power outage?

The exclusion key was really just a three-pole, double throw switch and you could use if for any function you wanted. I frequently used them to add intercom to six button sets that already had all buttons used.

From what I gather, unless this key was pulled up, the special signaling didn't occur. The can relays were probably isolated from the red phone's control until the exclusion key was lifted. Of course, you had to lift the handset before you could even access the exclusion key anyway.

Tip to ground would be hard to use in this application since tip is supposed to be at or close to ground potential to begin with. It would be like touching the two ends of a piece of wire together. I imagine that this was part of a modified ground-start trunk circuit in the CO, where instead of returning dial tone upon ground, it activated the array of relays and ringers all over town.

There's my $4.95 worth.

Ed Vaughn, MBSWWYPBX