Trunks are external lines

Trunks are external lines that provide the physical connection between a Norstar system and other systems in a private or public network. Trunks are numbered 001 to 156 in a fully expanded system. Norstar Modular ICS supports six different types of trunks:
• PRI trunks are used for incoming and outgoing calls over an ISDN network. PRI SL-1 lines can provide MCDN network functionality in a private network between other Norstar systems, Meridian 1 systems, or Business Communications Systems, if the appropriate software code has been installed.
• T1 trunks are digital trunks that can be configured to act as loop start, ground start, E&M, DID, or leased lines, depending on your requirements.
— DID trunks route incoming calls from the public network directly to telephones within Norstar, without an attendant.
— Loop start trunks handle incoming and outgoing calls
between Norstar and the public network.
— E&M trunks handle incoming and outgoing traffic between the Norstar system and the private network.
• BRI trunks handle incoming and outgoing calls between Norstar and an ISDN network.
• Target lines are virtual communication paths between trunks and telephones on the Norstar system. They are incoming lines only, and cannot be selected for outgoing calls. With 224 target lines in a fully expanded system, you can concentrate auto answer calls on fewer trunks. This type of concentration is an advantage of target lines.