"Trunk" denotes a connection to another network, e.g. the public telephone network. A connection to another network can be e.g. an "ISDN trunk", a connection to the Internet an "SIP trunk". Connections or trunks of the same type are combined to form groups. The trunks of a trunk group then have the same properties (such as the same connection protocol or the same rights parameters). The trunks of a trunk group are thus primarily "capacity expansions" from the user's point of view, with no further differences for their use. A trunk must always be a member of a trunk group.

Different trunk groups can be differently prioritized. The result is e.g. that calls are handled with preference given to a server-server connection. If this is not available, a lower-priority path is chosen, e.g. an ISDN trunk.