A trunk line is a high-capacity communication line that connects telephone exchanges. It carries data transmission between exchanges for local, long-distance, and international calls. If individual subscriber lines are the "capillaries," trunk lines are the "arteries" of the telephone network.
Trunk line capacity directly determines the number of simultaneous calls that can be handled. The "lines are busy" message heard during New Year's or major events occurs when call volume exceeds trunk capacity. Carriers predict peak demand by time of day using historical call data and design trunk capacity accordingly, but unexpected surges like those during disasters cannot be fully accommodated. During the Great East Japan Earthquake, calls to affected areas reached 50-60 times normal volume, and up to 90% call restrictions were imposed.
Trunk line technology has evolved over time. Early telephone networks used analog copper cables, carrying only one call per line. From the 1970s, digital multiplexing (TDM) enabled dozens to hundreds of simultaneous calls on a single line. Today, fiber optics are the standard, capable of handling tens of thousands of simultaneous calls per fiber. The spread of fiber optics has improved call quality and reduced costs, contributing to significant reductions in long-distance call rates.
As the migration from PSTN to IP networks progresses, trunk lines are also transitioning to packet-based transmission. NTT completed its landline IP migration in 2024, shifting from circuit-switching (dedicating a line per call) to packet-switching (dividing data into small packets over shared lines). Trunk lines connecting enterprise PBX systems to carrier exchanges are increasingly being replaced by SIP trunks. See Phone Number Structure for an overview of the telephone network.