Most groups weren't that big, but in Oklahoma City there was an incoming group of almost 100 "800" numbers which was routed through a step office (because of capacity reasons) because the 4A could not connect to and signal a subscriber line because it did not provide for the usual supervision. (off-hook, on-hook?) One of the local engineers figured out how to add one relay on the 4A that would allow it to supervise as a subscriber line. (he had first asked Bell Labs and then said it couldn't be done; aftet he reported to the Labs how he had done so a letter came out from the Labs describing the method.)
That saved a lot of holding time throught the network because every incoming call previously had to wait while the call progressed with step pulses.
I can't answer how it was done but I know it was done. The branch L.A. County Superior Court in Pomona was in GTE-land and served by a stepper in the 1970s. I would call the court at a busy time and hear it ratcheted through 10 clicks, a brief pause and then another 10 clicks before it returned a line busy signal. So, it would seem they had 20 lines in hunt.
I went to the University of Okloahoma for two years and Oklahoma A&M College (now Oklahoma State University for two years when Norman and Stillwater were both manual offices. The OU number was 900 and the Oklahoma A&M number was 1380.
When you asked the operator for the number you could hear them running the tip over the group busy jacks (which emitted a tone) and when they came to one that wasn't busy you could hear them running the jack laterally then to find an unbusy line. The operators were so adept that it took a while to realize what they were doing because it was so fast the delay was not much more than the usual busy test on a single line.
The group busy jacks I believe governed a strip of 20 jacks just to the side in the multiple. They corresponded to the level hundting on the SxS switches.
I'm not sure how it was done, but every step switch had a number of relays mounted on it for various logical functions, such as controlling the hunt for an empty line or returning a busy signal.
One waste of step by step was that this "logic unit", though small, was tied up on every switch for the duration of the call. If a call required seven digits, that was seven logic units tied up.
Please see the separate posts on this subject.
That undoubtedly had to be solved early because even in the early days there were many organizations that had more than ten incoming trunks.
One other observation about step vs. panel in the early days: As mentioned, step originally was not very sophisticated and required certain manual operations on the part of the subscriber, such as pushing a ringing key. Bell added some innovations to make the call progress as automatic as possible from the point of view of the subscriber--lifting the receiver initiated various actions and hanging up initiated various restore actions. It also converted step from local-battery to common-battery. (see the 1875-1925 history for more details on this).
Bell felt manual exchanges were more appropriate for medium and small sized cities cira 1910-1920. Operators were only on duty and paid when needed, as opposed to expensive automatic switchgear which sat idle when not needed. Manual switchboards were relatively cheap compared to automatic switchgear.
Bell also felt the 10x10 step by step matrix would be inadequate for city service due to the numerous exchanges, which is why they developed panel.
For the most smallest of offices Bell did want automatic switching since volume didn't justify having an operator on duty 24/7. But SxS wasn't up to the task in its earliest days. (Later a "community dial office" was specially developed and was a very popular offering.)
Bell and Automatic Electric agreed to share patents and Bell had AE build switch units for it for many years.
I believe at its peak SxS handled 49% of the lines in the Bell System. Interestingly, step was still strongly represented in the late 1970s (both in central office and PBX) despite its limitations and costs in handling DDD and Touch Tone*. But common control was still very expensive and only cost-justified in large high volume offices. Further, while SxS needed more routine maintenance than later units, the skill and debugging level was relatively easy as compared to crossbar which required more sophisticated training for the staff.
*For some reason, early step reversed polarity upon a connection. This would screw up early Touch Tone phones which were polarity sensitive (the tone pad). They had to change that. Also, adding a Touch Tone receiver front end to a step office was cumbersome and expensive, too. Bell found low-cost units that worked on PBXs weren't good enough for CO use. (Hard for us today to think about that kind of thing; we're so spoiled by high-grade electronics.)
The Bell System histories provide a good source of information for the above. Old "Bell Laboratory Record" magazines would be great to read. They're usually found in large urban or college library archives and require a request. If you're lucky enough to find an old set in open stacks, sit down and spread out and go through a bunch of them--fascinating!
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