Trunk line

From Skip2PBX-WIKI

When dealing with a PBX, trunk lines are the phone lines coming into the PBX from the telephone provider [4]. This differentiates these incoming lines from extension lines that connect the PBX to (usually) individual phone sets. Trunking saves cost, because there are usually fewer trunk lines than extension lines, since it is unusual in most offices to have all extension lines in use for external calls at once. Trunk lines transmit voice and data in formats such as analog, T1, E1, ISDN or PRI. See illustration here The dial tone lines for outgoing calls are called DDCO (Direct Dial Central Office) trunks.