PSTN (Public Switched Telephone Network) is the traditional analog public telephone network. Using copper cables to transmit voice signals, it has served as the foundation of telephone communications for over 100 years since Japan's first telephone exchange service began in 1890. It is a massive network composed of telephone exchanges and trunk lines.
The fundamental difference from VoIP lies in the communication method. PSTN uses circuit switching, where a dedicated physical circuit is established between caller and receiver when a call is connected, and that circuit is exclusively occupied during the call. While this ensures stable call quality, circuit utilization is inefficient since the line remains occupied even during silent periods. VoIP uses packet switching, dividing voice data into small packets transmitted over shared lines, which uses the line far more efficiently. Call charges, though, are set by each provider under its own pricing, so a change of switching method does not by itself make calls cheaper.
NTT carried out the migration from the PSTN to an IP network in regional phases starting in January 2024, completing it by January 2025. Called the "metal-to-IP conversion of landlines," this migration essentially doesn't change phone numbers or usage for subscribers. The underlying technology has shifted from analog circuit switching to digital packet switching. As part of the migration, ISDN data communication functions (INS Net 64/1500 data) were terminated, but voice calls remain available. My Line (pre-registration of call carriers) was also abolished.
NTT has continued to offer landline service since the IP migration. Landline subscriptions (subscriber telephone plus ISDN) fell from roughly 63 million in fiscal 1997 to about 14 million in 2023, and mobile and IP telephony took over as the main way people place calls. When setting up a fixed line from scratch, the practical fork is whether to draw a copper subscriber line or go with Hikari Denwa over a fiber connection. The Hikari Denwa migration guide walks through the steps.
A key strength of the PSTN was reliability: analog phones could keep working during power outages thanks to power feeding from the telephone exchange. The direct trigger for the migration was equipment aging - switch manufacturing had ended and maintenance parts were becoming hard to procure. Phone numbers and basic usage remain unchanged after the IP migration, but some supplementary services such as Dial Q2 were discontinued because they are not offered on the IP network. IP telephony over a fiber line, by contrast, goes silent in an outage once the optical network unit (ONU) and router lose power, unless a backup such as a UPS (uninterruptible power supply) is in place.