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. Because capacity is designed around peak demand in normal times, unexpected surges like those during disasters cannot be fully accommodated. According to the 2011 White Paper on Information and Communications published by Japan's Ministry of Internal Affairs and Communications, widespread call congestion occurred during the Great East Japan Earthquake, and carriers imposed call restrictions of up to 80% to 90% on fixed-line telephone service. The same white paper notes that in NTT East's case the rise in fixed-line traffic was smaller than on mobile networks, so those restrictions were lifted at a relatively early stage.
Trunk line technology has evolved over time. Early telephone networks used analog copper cables, carrying only one call per line. Digital multiplexing (TDM) later made it possible to bundle several calls onto a single line. As transmission moved to fiber optics, the number of calls a single cable can carry grew substantially.
As the migration from PSTN to IP networks progresses, trunk lines are also moving to packet-based transmission. The local switching equipment for landline service (subscriber telephone and INS Net) was moved to IP networks region by region from January 1, 2024, and NTT East states that this switchover has been completed (as of August 2026). Calls accordingly moved from circuit-switching (dedicating a line per call) to packet-switching (dividing data into small packets over shared lines). A trunk line between an enterprise PBX and a carrier exchange is called a SIP trunk when it is provided over an IP network. See How Japanese Phone Numbers Work for an overview of the telephone network.