When DID numbers were introduced, companies purchased blocks of sequential telephone numbers. The last four numbers would then provide the extension number for internal calls. However, external callers would be able to dial in directly to the right extension simply by calling the published number for that specialist or department.
In this new PBX-driven scenario, the telephone company would direct all calls to any number within that block to the PBX, which sat on the main number for the office. The PBX would then detect the last four digits dialed and switch the call through to the relevant extension.
Therefore, callers from hong kong telegram outside the building would get direct access, bypassing a switchboard or receptionist. Hence, “direct inward dialing.”
The area code on a telephone number directs calls to a specific location—either a city or an area. Once the call has been successfully connected through to the PBX for the dialed number, the telephone company’s responsibility ends. What happens to that call after it reaches the PBX is a private matter.
When it became possible to set up the PBX so that calls to a specified number got forwarded out of the building—either to a home-based worker’s telephone or to another office building— businesses used leased lines to channel private calls to other buildings.
However, leased lines are expensive to run. Business services entrepreneurs came up with a way to create virtual private networks over public lines. This provided the privacy of a private network while only requiring per-call charges rather than the hefty fees of a private cable.
This expansion of private networks beyond office walls was only made possible when office telephone systems switched over from analog to digital technology. Voice data was digitized and run over the data network, then tagged so that it could be identified as a separate channel, even though it traveled along the same wire as data.
That data packet tagging is precisely how the internet works. Companies could then make one of the lines managed by the PBX lead to the internet, so a particular extension number could easily be assigned to one of the telephones in the building or to another line outside of the building.