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, resulting in higher efficiency and significantly reduced costs.
NTT completed the migration from PSTN to an IP network in January 2024. 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.
PSTN's IP conversion is a global trend. BT in the UK is advancing plans to completely abolish PSTN by 2025, and Deutsche Telekom in Germany has completed a similar migration. NTT will continue landline service after the migration, but long-term service reduction is expected as landline subscriptions decline. Those considering migration to Hikari Denwa can review procedures in the Hikari Denwa migration guide.