Circuit switching dedicates a communication circuit between caller and receiver for the entire call duration. It's the foundational technology of traditional landline (PSTN) and 2G/3G mobile networks, with exchanges controlling circuit connection and disconnection. A "dedicated circuit" does not mean a whole copper pair is monopolised: the exchange assigns each call a fixed slice of transmission capacity (a 64 kbps speech path on digital networks) and keeps any other call off it until the call ends.
The advantage is guaranteed call quality. Dedicated circuits prevent interference from other communications, minimizing voice delay and jitter. The high reliability expected of emergency calls (110/119) was likewise built on the stability and priority handling worked out in the circuit-switched era, and that design thinking carried over as the network moved to IP.
The disadvantage is low circuit efficiency - silent periods during calls still occupy the line, wasting bandwidth. VoIP's packet switching divides data into small packets sharing lines, improving efficiency.
NTT East and West moved their PSTN onto an IP network in stages from January 2024 and finished the switchover by January 2025 - the work known as "PSTN migration". Phone numbers and handsets carried over unchanged, so for people who only make voice calls almost nothing about the service changed. On mobile networks, 4G VoLTE carries voice as packets as well, so reserving a speech path per call has retreated from the main voice networks. One point that trips people up in practice is treating per-minute billing (the "X yen per three minutes" model) as something circuit switching requires. Time-based charging took hold while calls really did occupy a line, and it simply continued as a pricing structure after the move to IP. See VoIP basics for circuit vs packet switching comparison.