Telephone Systems Terminology, DID Trunks

Direct Inward Dialing, allows a telephone number to be assigned to a telephone systems telephone or other device enabling callers to place telephone calls directly to the telephone set or telecommunication device. DID trunks allows one way calling only, no outbound calls can be made over a DID trunk.

DOD trunks (Direct outward Dial) a counterpart of DID is a technology used with PRI ISDN Trunks that allows phone calls to be placed outbound over the PRI Trunk.

A DID trunk is the path between the business telephone system and the telephone switch of the telephone service company which provides the DID Trunk, (ie AT&T, Verizon, TelePacific, Cbeyond, etc).

DID telephone numbers are digits sent to the business telephone system from the telephone carrier. The telephone system translates the digits sent from the carrier.

The term sent is best described as, the digit dialed are recorded and once the path between the telephone system of the carrier and the business telephone system are connected the recorded digits are played back over the path. These digits are than interpreted and translated by the business telephone system enabling the telephone system to route the call to the proper telephone or other telephone device. Touch tone dialing is about sound with each of a telephone set dial pad producing a different pitch or length of a sound enabling telephone switches to decode all digits as individuals.

To understand the next portion of this explanation think in terms of, lifting the handset of a telephone set connected to a business telephone system and hearing the intercom or extension dial tone. As you already know you are able to dial a few digits and connect with any other telephone user connected to the telephone system at you business.

The telephone switch of the telephone carrier which provides the DID trunks is also able to effect the same with a business telephone system, provided the path is connected (A DID Trunk, which is connecting your business phone system with the carrier’s switch). Using one of the signaling technology, types Wink Start, Ground Start, or Loop Start, the Telephone Switch of the Carrier will signal your business telephone system and receive the dial tone of your business telephone system and send the digits required. (play the numbers recorded)

These actions by the telephone carrier’s switch and your business telephone system effects the same action as if  someone lifted the handset of a telephone within the business, and upon hearing the intercom tone or extension dial tone created by your business telephone system and began to dial an extension number.

The routing of the DID number is programmed by the business telephone systems technician, the programmer will sit up a routing table instructing the telephone system that when the carriers switch sends a string of digits that business telephone system will interpret them as (XXXX = YYYY) meaning that the digits sent will be translated into the following extension number of your business telephone system.