Direct inward dialing (DID) on small business telephone systems

Direct inward dialing (DID) is a feature that small business telephone system uses with their private branch exchange (PBX) systems. In DID service the telephone company provides one or more trunk lines to the customer for connection to the customer’s PBX and allocates a range of telephone numbers to this line (or group of lines) and forwards all calls to such numbers via the trunk. The service allows direct inward call routing to each extension while maintaining only a limited number of subscriber lines to satisfy the average concurrent usage of the customer. This helps small businesses in sharing resources and avail numerous connection with just a single connection (for e.g. 0800 numbers / 0800 uk numbers).

A DID requires a DID trunk to be backed by a customer premise equipment. The central office equipment detects the power state of the line and disables service if the circuit is not powered up. This is the reverse arrangement from standard plain old telephone service (POTS) lines which are powered by the central office.

A DID system is very much used by fax servers. A telephone line is terminated in a computer running fax server software and fax modem cards. DID enables recipient to have an individual fax number even though there is only one fax machine. DID system also works in align with VoIP. To reach users with VoIP phones, DID numbers are assigned to a communications gateway connected by a trunk to the public switched telephone network and the VoIP network. The gateway routes and translates calls between the two networks for the VoIP user.