Voice calling is real-time voice conversation over telephone or internet lines. The most fundamental phone function, it underpins landlines, mobile phones, VoIP, and the audio component of video calls.
Two main technologies exist. Traditional circuit switching dedicates lines for voice signal transmission. Packet communication (VoIP) digitizes voice and sends it as internet packets. Current 4G/5G phones use VoLTE (Voice over LTE) for packet-based voice calls.
Mobile plans distinguish "voice SIM" (with phone number for regular calls) from "data SIM" (data only, no phone number calls but app calls like LINE work). MVNOs typically charge several hundred yen more monthly for voice SIMs.
Voice quality depends on codecs. HD Voice (AMR-WB) uses wider frequency bandwidth for clearer audio than legacy codecs. See LINE call vs phone call for voice call type and quality comparison.