In telecommunications, trunking is a method for a system to provide network access to many clients by sharing a set of lines or frequencies instead of providing them individually.This is analogous to the structure of a tree with one trunk and many branches.runking 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.