In reference to the posting about a PBX needing a good home, here is some historical background material:
For younger readers, the 556 was a cord switchboard for moderately sized dial PBX's. The 556 was installed along with a dial switch; various types of dial switches could be used. Up to about 120 lines and 20 trunks could be handled by a single switchboard, two switchboards could be placed side by side to handle double that.
The 556 was similar to the popular 555, the 555 was a manual PBX. Both were released after WW II and featured improved circuits and easier maintenance. Because the 555 was manual (the attendant handled all calls), it was a stand-alone unit (see below).
In an installation with a 556, extension users could dial calls between extensions, and where authorized, dialed 9 to make an outside call. The PBX attendant answered incoming calls and routed them to the desired extension. The attendant also provided other assistance as needed, such as placing toll calls, keeping toll records, and screening calls.
In the 1960s, customers with modest dial PBX's began to replace their cord switchboards with new desktop consoles with more automated features. For instance, on a cord switchboard, the attendant had to take down the cord pair at the conclusion of a call; on a console, disconnection was automatic. On a console switchboard, an incoming call could "camp on" to a busy extension and be automatically connected as soon as the extension was free. The automation allowed the attendant to have other duties, and many doubled as the receptionist.
In the 1960s onward, key systems grew more sophisticated and replaced PBX's in small installations. Key systems didn't require a dedicated trained PBX attendant.
A Bell System marketing pamphet from 1962 says the following:
"The 556 switchboard gives your business all these advantages:
FAST, SEMI-AUTOMATIC OPERATION . Your telephone attendant completes calls quickly and easily with angle-mounted cords and keys. . Pushbuttons and positive visual control signals assure fast, accurate communications.
EXPANDABLE-VERSATILE . Expandable to match the capacity of your dial communication system. . Works with all kinds of trunks, tie lines and auxiliary PBX equipment.
MODERN-CONVENIENT . Attractive, low-silhouette design. . Can be custom finished to blend with your decor. . Can be readily installed as an integral part of your reception area. . Compact cord and key shelf add comfort and operating convenience.
MORE THAN PAYS ITS WAY . Your attendant handles more traffic in less time. . Simplified controls assure efficient call handling and better customer relations. . Prompt, reliable maintenance at no extra cost. . Readily expanded or altered without costly delays . No capital investment.
The 556 Switchboard...gives you faster communications for the efficient operation of all your business functions-- Administration, Purchasing, Production, Distribution.
Features: . conference calling . emergency service . night answering . tie-line service . connects to your paging system . expands with your needs"
- * * A Bell System marketing brochure for the manual 555 from 1962 focused on cost control:
"PERSONALIZED HANDLING OF INCOMING CALLS . All incoming calls are screened by your attendant. . She completes calls quickly with simplified switchboard equipment. . Priority handling of special calls easily arranged. . After-hours calls can be routed to any "inside" telephone you select.
CONTROLLED OUTSIDE CALLING . All "outside" calls are made through your attendant. . Handles up to 13 outgoing calls at once. . After-hours "outside" calling is easy to arrange. . Permits accurate accounting of all calls. . control all calls. . maintain accurate accounting."
- * *
The manual 555 obviously saved on the rental of dial switchgear, but this was offset by the expense of additional PBX operator time and the delay of an operator completing all calls. Since an operator was required anyway to handle incoming calls, the additional time to handle intercom and outgoing calls could be modest--it depended on the traffic of a particular installation. Sites with heavy intercom traffic would likely need a dial PBX, especially if that traffic continued after the switchboard was closed for the day. As mentioned, the 555 was a very popular switchboard, so plenty of sites were content to be all manual.
A single 555 could handle up to 120 extensions and 14 trunks. Two switchboards could be installed side-by-side to double that.