A voice call is a real-time spoken conversation carried over a telephone or internet line. It is the most basic thing a phone does, and it sits at the base of every telephone service: landlines, mobile phones, VoIP, and the audio side of a video call.
Two families of technology carry that audio. Circuit switching holds a single path open for the length of the call and sends the voice signal along it. Packet communication, or VoIP, turns the voice into digital data and sends it as packets. On mobile networks, 4G carries voice as packets using VoLTE (Voice over LTE), while 5G both falls back to VoLTE and uses VoNR (Voice over New Radio), which carries voice natively on the 5G network. The 3G networks that carried voice by circuit switching were shut down one by one: SoftBank ended its service in April 2024, and NTT Docomo closed FOMA at the end of March 2026.
Japanese mobile plans separate a "voice SIM" from a "data SIM." A voice SIM comes with a phone number and places and receives ordinary calls. A data SIM handles data only and cannot dial or answer on a phone number. Calls inside an app still work, so a data SIM is fine if app messaging covers everyone you contact. The catch is that SMS verification codes will not arrive unless the contract includes an SMS option, and that you cannot reach emergency numbers such as 110, which is why even an MVNO normally prices a voice SIM above a data SIM.
Call quality turns on the codec, the scheme that compresses the audio. HD Voice (AMR-WB) uses a wider frequency range than the older voice codecs, which changes how easily consonants can be told apart. The wider range only applies when both ends support it; if one side does not, the call falls back to the narrower audio. LINE calls versus phone calls compares the types of voice call and their quality.