The most "efficient" numbering plan (least amount of equipment) would be to assign extension numbers in three 3-digit ranges:
211-200 311-300 411-400This arrangement provides a total of 300 lines, enough to accommodate
250 apartments with spares for other uses (office, maintenance room, loading dock, etc.).Advantage: This plan requires only three sets of switches: linefinders, first selectors, and connectors. By contrast, the hypothetical 4-digit plan that "one person" suggests would require four sets: linefinders, first selectors, second selectors, and connectors. As you note, a four digit code [is] inherently more complex than a three digit in an SxS environment."
Advantage: This plan avoids two SxS taboos: subscriber numbers beginning with 0 or 1:
- Initial 0 is taboo because "experience has shown that in a single- office system it is not advisable to use subscribers' numbers commencing with the numeral zero (0) unless absolutely necessary to secure the desired capacity" [1]. And, of course, zero is traditionally used to reach the local operator, if any.
- Initial 1 is taboo because an SxS (or any other rotary-dial) switch can't distinguish between an intentionally-dialed 1 and a false switchhook depression.
Disadvantage: It's not possible to match subscriber numbers to apartment numbers (although, as PAT notes, this might not be a good idea anyway).
[1] Kempster B. Miller. "Telephone Theory and Practice" vol. 3 "Automatic Switching and Auxiliary Equipment." New York: McGraw Hill, 1933, p. 129.Neal McLain