Making and receiving calls between internal extensions is one thing, but if you want to receive and make calls to the outside world, you need at least a trunk to the outside world.
How to Configure Phone Lines
ISDN Digital Circuits
The ISDN network consists of numerous units; what we would like to introduce among them in this section is a digital circuit, a level of service of the ISDN. Let us see how can the ISDN manage these digital circuits and the data transfer.
ISDN can work if there are two types of channels, the B-channel (bearer channel) and the D-channel (data channel). The B-channel is used for carrying the data content through the network, the D-channel is for carrying the signaling and supervisory information.
According to the number of the B- and D-channels, two types of interfaces can be distinguished: the Primary Rate Interface (PRI) and the Basic Rate Interface. The Basic Rate Interface always consists of two B-channels and one D-channel. BRI is used by individual users and small companies.