Network Sales and Services Handbook (Cisco Press Networking Technology)

A line is an end point from a CO or a PBX and carries a single conversation. A trunk interconnects two switching systems, such as between LEC central offices, or a LEC CO switch and a customer PBX. Trunks bundle several lines, carrying multiple simultaneous conversations and are used for call switching and routing.

Direct Inward Dialing

Direct Inward Dialing (DID), refers to a service whereby a caller can dial a 10-digit number from outside a switch and reach a specific individual without an operator (live or an automated attendant) intervention. In the context of a PBX, DID is a feature that must be configured to enable the direct dialing of an extension.

Direct Outward Dialing

Similar to DID, Direct Outward Dialing (DOD) enables a caller behind a switch or PBX to place outbound calls without an operator or automated attendant assistance. This was necessary in the early days of corporate telephone systems where operators screened incoming and outgoing calls, such as prohibiting employees from making international long-distance personal calls; these restrictions can now be configured on a corporate switch or PBX.

Tie Lines

Tie lines, or tie trunks, are dedicated private line, point-to-point, circuits used to connect two voice facilities; such as a dedicated trunk between two organization's PBXs, in different locations. There are no geographical limits to implementing tie lines, however, cost needs to be a consideration as most network service provider private line services are distance sensitive with a billable cost-per-mile.