Answering Service

Many telephone subscribers—mostly medical doctors--required 24-hour answering services. The local operating company did not provide this service. This opened up opportunities for private “Answering Service” businesses. These services forwarded out-of-hours calls to a private answering company.

In 1955, a specially designed “answering service” switchboard, called the 557, was developed. This switchboard had the same modular construction first used with the 555 and 556 switchboards introduced in 1948. This modular design made these switchboards easy to service and maintain.

An answering service bureau would receive calls from many locations—for example, various medical offices all over a city. To transfer calls to the answering service, the receptionist at the doctor’s office would operate a transfer key at the close of each business day. As long as this key remained in the transfer position, all incoming calls to the doctor’s office would be routed to the 557 switchboard at the answering service.
To reduce the facility requirements of these answering service locations, a “Secretarial Concentrator/Identifier” system was installed. This device would accommodate 100 transferred lines over six facilities. We have a Concentrator/Identifier connected to this board.

Multi-line Service and PBXs

In many situations customers would need more than one line. Telephone sets and apparatus were developed that allowed multiple lines to be answered on a single telephone set. “PBX”—or private branch exchange—is a term that refers to any type of telephone switch that is located on the customer premises and provides service between telephones inside a single building or internal to a school, factory, or office.

756 PBX

By the early 1950s, Bell System researchers were eager to develop a larger crossbar switch. The 755 crossbar PBX—introduced years earlier, in 1938—was an excellent switch but could handle only up to 20 lines. For larger dial PBX systems, customers were still using either 700-series or 740-series step-by-step PBXs, based on 1920s technology. Although there had been incremental improvements to these step-by-step PBXs, their basic designs were obsolete by the early 1950s. Compared to crossbar, step-by-step PBXs were very slow. They also required frequent maintenance and were difficult to install and reconfigure.

The 756 PBX was introduced in 1956. Two versions were available, one to serve 40 lines, another to serve 60 lines. The 756 PBX was an exceptionally clever design. It was the first PBX to work just like a very small central office switch, and it offered many new features which would later become standard with the all-electronic PBXs of the 1970s and 1980s. The 756 was also the first PBX to use the new wire-spring—these provided redundant paths for critical circuits, which made the 756 far more reliable than older PBXs. The 756 was also relatively compact and much quieter than the older step-by-step PBXs. This made customers happy. For the first time, a dial PBX did not need to be installed in a special equipment room. For its day, the 756 PBX had lightning-fast performance.

Dial 750 PBX

This is a 750 PBX, introduced by the Bell System in 1929. It was available in both 8-line and 15-line versions. It was developed originally for residential use but soon proved popular with small businesses. This was the first PBX system that did not require an attendant to direct calls. Each telephone used with the 750 PBX had five buttons on the base. One button was used for intercom dialing, another for hold, and the remaining three to select up to three central-office lines. In many ways the 750 PBX was the forerunner of key telephone systems commonly used in businesses from the 1950s to the 1980s.

BD 72 Portable Military switchboard

Frontline battle zone communications were very important to the war effort in World War II. Although it doesn’t look like much, this WWII BD 72 Field Switchboard was a self-contained, portable communications hub and was critically important to military operations. It is a manual magneto system that used EE-8 field phones connected with wires strung in the air or laid on the ground.

Dimension 400 PBX

In 1975, the Bell System introduced the Dimension 400 PBX. This was the first microprocessor-controlled, solid-state PBX and was the first Bell System PBX with no moving parts. Many new features were available with this technology. The Dimension 400 PBX was also much easier to install and maintain than previous electromechanical PBXs.

755 PBX

Introduced in 1938, the Western Electric 755 PBX was the first small automatic telephone switch to use crossbar technology. This made it very fast to operate, very reliable, and relatively compact. The 755 was made in both ten-line and twenty-line versions, and was often used in small offices, schools, or factories. Each station had a special telephone with both a dial and six buttons (or in telephone lingo, “keys”). You could select an internal line to dial directly to any other telephone in the system—without assistance from an attendant. Or, you could select one of four outside lines to call outside the office. The sixth button was a hold feature. Typically, an attendant was on duty at a front desk to answer and route incoming calls. The 755 PBX had an exceptionally long life and was manufactured from 1938 to 1964, though many remained in service well into the 1970s and 1980s.


This equipment is a Stromberg-Carlson PBX. Using their X-Y system equipment design, this unit supports 40 extension users. It has a very unusual numbering scheme. Due to its plunger type design the brush contacts operate horizontally on a bank of contacts, making the single switch move further with each dial pulse. Compared to some of the other PBXs on display, the Stromberg-Carlson is not all that elegant. For one thing, it’s very noisy and slow. Also, it has no dial tone and buzzes the called telephone instead of ringing it.

50-type Key Telephone Unit, used with 1A Key System

In telephone lingo, “key” refers to the type of lever switch originally developed for switchboards. During the economic boom of the 1920s, various key systems were developed for business use—their chief advantage was that the person using the telephone could select the telephone line they wanted to use without asking an attendant to select the line for them. Even during the lean times of the 1930s, key systems were developed for business applications. Instead of separate keys in wooden boxes, newer business telephones had pushbuttons to select multiple telephone lines, plus modern features still used today such as putting calls on hold, signaling other stations, and an intercom feature.

A tremendous advantage of key systems is that they do not require an attendant—this lets you use the telephone system after hours or during busy times when the front-office receptionist or attendant may be otherwise engaged. In 1938, the first modularized key system was developed along with a matching 300-series key-system desk telephone. This was called the 1A Key Telephone System and was very popular right from the start. It was relatively easy to install and service and was the forerunner to the later 1A1 and 1A2 Key Telephone Systems, some of which are still used today.

The Horizon PBX

The Horizon PBX was just one of the electronic customer premise switches offered by the Bell System.

507A Cordless PBX

Not all switchboards used cords. Based on earlier designs, the 507A and 507B cordless PBXs were introduced in 1952. The two units were similar. The 507B was slightly larger, with five incoming trunks. The attendant could route each of these incoming lines to one of twelve internal stations. Or, an inside caller could request a connection to another internal station.

The 507B console was much smaller than a cord PBX. It could fit easily on a reception desk. But it was really only practical for small businesses with relatively light telephone traffic.

The 507A and 507B cordless PBXs were the last of their type manufactured by the Bell System and were discontinued in 1966. By that time, 1A2 key system installations were fast becoming the most popular telephone system for small offices that had previously used cordless PBXs or manual cordboards.

505C PBX

The 505 PBX was originally introduced in 1909. It was the first cordless PBX to offer both central office and intercom switching in a single cabinet and was relatively easy to use. In fact, all cordless PBX designs shared similar features—including the 507-series PBXs, which were manufactured until 1966.

608 Switchboard

The last Western Electric PBX cord boards were the 608 switchboards. This series was developed in the late 1950s and introduced in the early 1960s. It was the last cord board introduced, and the first to use a plastic and fiberglass shell instead of polished wood as used on traditional switchboards.

The 608 cord board did everything the old switchboards could do and a lot more. Because it was designed as a universal replacement for older switchboards, it could serve anywhere from just a few lines up to 2400 lines. It had plug-in keys and relay units—this made the 608 easy to install, configure, and service. It was also the first switchboard to have a lower shelf. This made the board easier to use.

Fast response to customer complaints (called “trouble reports” in telephone lingo) and quick repair of the problem was very important. Modular design of modern telephone equipment permitted fast replacement of defective components.

Long Distance 3 CL and Frederick & Nelson boards

As the industry moved toward customer-dialed calls, the need for operator assistance was reduced, and this type of cord board was eliminated. This particular board, a 3CL type, was the last of its kind used at Pacific Northwest Bell. There were many of these boards in nearly every city served by PNB. As the transition to customer-dialed calls took place, circuits that required operators were centralized to this particular board (which was located in Portland, Oregon). Once the conversion to customer-dialed service was completed in the 1980s, this board was retired and subsequently donated to our museum.

#3 ESS

This is a number 3 Electronic Switching System (ESS). It was designed in the late 1970s to serve smaller communities that until that time were mostly served by step-by-step systems. The number 3 ESS could serve up to 4500 customers. It was typically installed in what was called a CDO—a community dial office. This type of office was much smaller than a central office and was unattended. The number 3 ESS could also be used as a large private branch exchange.

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PICTURED: (above, beginning of article) (left) This is a 557A Answering Service Switchboard. Customers would switch their line to the answering service for out of hours calls. These services were provided by private answering service businesses. (middle, right) To eliminate the need for massive cable facilities to serve these private businesses, concentrator equipment to allow multiple customer lines to be directed over just a few pairs of wires were developed. These photos are of the answering service location equipment and the telephone office equipment.
PICTURED: (throughout article) The images on this page are described fully in the text. Click the images to enlarge them.