Lines,  Trunks, Channels & Circuits

Your local telephone exchange carrier (Verizon, etc.) can deliver telephone calls via discrete lines, analog PBX trunks or digital ISDN channels. Digital channels provide simultaneous direct inward dialing, outbound dialing and powerful features such as Caller ID Name delivery and fast call delivery. 

In terms of capacity, 1 line = 1 trunk = 1 channel. It's the type of call and the added features they deliver that are different. 

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LINE = one dialed phone number rings one device at the end of a copper pair. Ex: Your home telephone line.

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TRUNK = many dialed numbers ring one device at the end of a copper pair. That rung device re-rings various extensions based on the dialed number.

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CHANNEL = a digital trunk. Channels are digitized and transmitted over 1 or 2 pairs of copper along with other channels. The copper carrying the channels are called a circuit. An ISDN BRI circuit has 2 voice channels, an ISDN PRI circuit has 23 voice channels. Channels over ISDN provide the most features for voice communications.

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CIRCUIT = a multi channel facility such as ISDN BRI,  ISDN PRI or a T-1. 1 or 2 pairs of copper that carry several simultaneous digitized calls. Each call consumes 64K of bandwidth from the circuit.

For a superior telephone system, nothing beats the all digital dial tone of ISDN.