"The last three pairs (actually two and a half) are the leads used to permit remote control of the set by a 4A speakerphone system."

And 3-type speakerphones, too.

I worked in a hospital that had over 12,000 key sets, and our rule was "always a home run, and never a bridging clip." I think there might have been 100 bridging clips in the entire hospital. They are the Devil's tool.

Arthur P. Bloom
"30 years of faithful service...15 years on hold"