A telephone line is a communication circuit for transmitting voice calls. Japan has four main types: "analog lines" using NTT's copper (metal) wires, "ISDN lines" for digital communication, "fiber optic lines" (Hikari Denwa) using optical fiber, and "mobile network lines" via wireless base stations.
Analog lines are the traditional method since Japan's telephone service began in 1890, transmitting voice signals directly through copper wires. They work during power outages via exchange-supplied power, but NTT has been migrating analog infrastructure to IP networks ("PSTN Migration") since 2024. Users' phones and numbers remain unchanged, so most people won't notice the transition.
Fiber optic lines (Hikari Denwa) convert voice into IP packets transmitted through optical fiber. They offer lower basic fees (about 550 yen/month) and higher voice quality than analog. However, they don't work during power outages, so public phones or mobile phones should be kept as backup.
Mobile lines use 4G (LTE) and 5G wireless networks for voice calls. While traditionally circuit-switched, VoLTE (Voice over LTE) now transmits voice as IP packets, offering better quality and faster call setup than 3G. See VoIP basics and Hikari Denwa migration guide for more details.