... like making a ten button set work on a 25 pair cable (three pairs per line, eight lines plus hold and ICM = 10 buttons).
Well, sort of...
Actually, it's two and a half pairs per line key, of which there are 9, whether they are used for CO/PBX or ICM service.
(The unused "balance" leads of line keys 2, 3, 4 & 5, generally left insulated and stored in a 56X-series phone, are used as the "A" leads for line keys 9, 8, 7 & 6, respectively.)
So, counting the conductors, we have:
T,R,A,LG,L times 9 keys is 45 conductors.
A1 ground = (Orange/White) = 1 conductor
Ringer pair (Yellow/Slate) = 1 pair = 2 conductors
Buzzer pair (Yellow/Orange)= 1 pair = 2 conductors
Total conductors = 50
In a Western 2830 set, the lamp grounds for line keys 1 through 7 are dedicated to mounting cord conductors. (In ITT sets, I seem to recall that the lamp grounds were "bunched" on terminal screws, allowing them to be separated and used for other applications.) The last two lamp pairs (LG & L leads for line keys 8 & 9) and their associated mounting cord leads are connected on terminal screws inside the set, so that they can be moved and used to provide the four leads needed for 3-type speakerphone service, (T1, R1, 1T & 1R) in the absence of a separate cord between the set and the 55-type control unit. If the control unit is hidden in a wiring closet, the mounting cord leads are used, because the assumption is that the 25-pair cable is threaded through walls and ceilings, back to the equipment. If the control unit is stuck under the victim's desk, then a separate 10-conductor cord could be used to provide the leads.
If common bell cut-off was specified, the Y/Sl leads could be re-routed in the closet through the K1 relay contacts in the 55-type control unit, or they could be fed through extra conductors in the 10-conductor cord at the desk location.
I often ran a 25-pair and an extra 4-pair in places in offices that seemed likely to need a few more pairs, especially if the cables were being run in conduits. The amphenol hung out of the wall plate, and the 4-pair was left coiled inside the wall, where it could be terminated later on a 44A block or whatever piece of equipment for which it was required.
This turned out to be a good idea in offices where the system was migrated to a ComKey 416 system, which required that the CO T&R feeds be fed to the primary stations on separate pairs. (i.e. not stealing the Violet leads)
The ground returns for the ringer and the buzzer could use the A1 ground, in a pinch, and that would also provide an extra pair. The unbalanced condition of using half-pairs for ringer and buzzer did not cause any appreciable cross-talk on relatively short runs (under 100 feet) from the set to the closet, where the unbalance was corrected to a balanced pair via the cross-connection field.
