DID refers to direct inward dialing. From a caller’s point of view, this service is in place if the caller can dial a 10-digit number from the outside, and reach a specific individual without operator (live or automated) intervention. Thus, Centrex normally inherently supports this capability without any additional configuration—everyone already has their own telephone number. A true key system (where telephone numbers are normally shared) can only do it if any given telephone number has only a single appearance.

But DID is usually referred to in the context of a PBX. It is a specific PBX feature that must be enabled and configured, with elements set up both within the PBX and also with the telco. Consider as an example a new site intended to support 1100 employees, each with his or her own telephone connected to a PBX.

The first step in arranging DID is to reserve the telephone numbers for all those employees. Let’s say that the main company telephone number is 555-1234. The Telecommunications manager will request a block of DID numbers from the telco, probably about 2000. The telco might say, Your DID numbers are 555-2200 through 555-4199. Notice that while there is a good chance the block will have the same exchange as the main number, it probably will not include (and one would not want it to include) the main number. The company will pay for these numbers on a monthly basis, but nowhere near as much as would be required for actual telephone lines. So far, the only thing arranged is the reservation of the block of numbers themselves. These numbers will not be given out by the telco to anyone else. The Telecommunications manager will assign each employee one of the numbers in the DID block.

Next, the Telecommunications manager must determine how many trunks (or DID lines) in the trunk group will be required to support the calls from outside to the company’s employees. These are inbound only, and are in addition to the normal in-out or inbound trunks that serve the main operator, so they must be engineered to a very low level blocking indeed. With DID, the telco passes on to the customer PBX the responsibility of handling answer supervision (e.g., busy signals).

So, if an external customer calls Jane at extension 2313, the customer will dial 555-2313. The telco CO will seize the next available trunk in the DID group (if no trunk is available, the caller will receive a busy signal), and signal along it that there is a call for extension 2313. At that point, if extension 2313 is busy, the PBX must deal with it; the CO is merely passing along the signals. Possible PBX actions include forwarding to a message center, generating a busy signal, or forwarding the call to a specified alternate extension.

DID is most often used to reduce or eliminate the manpower required for a central answering position. The more calls that customers can place directly, the fewer must be answered by the company operator. On the other hand, some companies prefer to have all incoming calls answered by someone trained in how that company wants its telephones to be answered (e.g., Thank you for calling Kay’s deli! How can I help you vs. Hello?). It would generally be a mistake for a Telecommunications manager to make decisions regarding whether DID is to be implemented without consulting with company management.