DDD is a North American Numbering Plan term considered obsolete since completing a call in any manner other than direct dialing became rare. In the United Kingdom and other parts of the Commonwealth of Nations, the equivalent terms are or were "STD", for subscriber trunk dialling, and "ISTD" for international subscriber trunk dialling.
The 11 cities and their area codes at that time were:
Many other cities could not yet be included as they did not yet have the necessary switching equipment to handle incoming calls automatically on their long distance circuits. Other cities still had either a mixture of local number lengths or were all still six-digit numbers; Montreal and Toronto, Canada, for example, had a mix of six- and seven-digit numbers from 1951 to 1957, and did not have DDD until 1958. Whitehorse, Yukon, had seven-digit numbers from 1965, but the necessary switching equipment was not in place locally until 1972.
The card sorter of the 4A/CTS (Number 4A Crossbar / Card Translator System) allowed six digit translation of the central office code number dialed by the customer. This determined the proper trunk circuits to use, where separate circuit groups were used for different cities in the same area code, as in the case of Oakland and San Francisco. The new device used metal cards similar in principle to computer punch cards, and they were rapidly scanned as they fell past a light beam. On busy days, it sounded like a machine gun firing. CTS machines were called 4A (Advanced) if the translator was included in the original installation, and 4M (Modified) if it was added later. A 1970s version of 4XB, the 4A/ETS, used a computer to translate. For international dialing, TSPS provided the extra computer power.
The reach of DDD was limited due to the inefficiency and expense of switching equipment, and the limited ability to process records of completed calls. One obstacle was that the majority of switching gear did not provide Automatic Number Identification. Common control switches such as 1XB switch were fairly quickly retrofitted to provide ANI, and most 5XB switch were initially installed with ANI services. Panel switch were eventually retrofitted, as were some Step by step that were not scheduled for immediate replacement.
As this and other improved technologies became available, as well as Automatic Message Accounting (AMA) computers to process the long distance records into customer bills, the reach of DDD was slow in the 1950s, but quickened in the early 1960s. Electronic switching systems with stored-program capability allowed electronic processing of the dialed digits, referring to electronic memories to determine call routing, and this has reached the state-of-the-art with digital telephone exchanges which are basically specialized computers that route voice traffic from one "peripheral" to another as digital data. Call routing can now be done based on the area code, central office code and even the first two digits of the line number, although routing based on digits past the central office code is usually limited to cases of Competitive local exchange carriers and number portability.
The key to the new system was two-stage multi-frequency pulsing. The Outgoing Sender sent its Class 4 Toll Center an off-hook signal as usual, received a wink as usual as a "proceed to send" signal, and outpulsed only a special 3-digit (later 6-digit) access code. The Toll Center picked a trunk through the Long Distance network to the gateway office, which sent a second wink to the originating office, which then sent the whole dialed number. Thus the toll switching system needed no modification except at the gateway. The international trunks used Signaling System No. 5, a "North Atlantic" version of the North American multi-frequency signaling system, with minor modifications including slightly higher digit rate. European MF systems of the time used compelled signalling, which would slow down too much on a long trans oceanic connection.
In the 1970s, Toll Centers were modified by adding TSPS. With these new computers in place, digit storage in the toll system was no longer a problem. End offices were less extensively modified, and sent all their digits in a single stream. TSPS handled the gateway codes and other complexities of toll connections to the gateway office.