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.
The advantage is guaranteed call quality. Dedicated circuits prevent interference from other communications, minimizing voice delay and jitter. Emergency call (110/119) reliability stems from circuit switching's stability.
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's "PSTN Migration" from 2024 transitions circuit-switched networks to IP. 4G VoLTE also processes voice as packets. Circuit switching is gradually disappearing, though users' numbers and phones remain unchanged. See VoIP basics for circuit vs packet switching comparison.