Similar schemes were used in the Americas and in some European countries including Spain. This allowed direct current E and M signaling, or dial pulses, to be sent between electromechanical switches over a digital carrier which did not have DC continuity. Bell System installations typically had alarm bells, gongs, or chimes to announce alarms calling attention to a failed switch element. A trouble reporting card system was connected to switch common control elements. For example, if a dial customer calling from TAylor 4725 dialed a number served by a manual exchange, e.g., ADams 1383-W, the call was completed, from the subscriber’s perspective, exactly as a call to LEnnox 5813, in an automated exchange.
The term telephone exchange is often used synonymously with central office (CO), a Bell System term. All central offices within a larger region, typically aggregated by state, were assigned a common numbering plan area code. Every fractional second cut off of call set up time meant fewer racks of equipment to handle call traffic.
Telephone switches are usually owned and operated by a telephone service provider or carrier and located in their premises, but sometimes individual businesses or private commercial buildings will house their own switch, called a PBX, or Private branch exchange. Switching, transmission and billing equipment may be slaved to very high accuracy 10 MHz standards which synchronize time events to very close intervals. Crossbar offices used more shared, common control circuits. For example, a digit receiver (part of an element called an Originating Register) would be connected to a call just long enough to collect the subscriber’s dialed digits.