A ringtone is the sound that plays to alert you to an incoming phone call. In the landline era, the classic "ring-ring" bell was the only option, but with the spread of mobile phones, electronic tones, melodies, and full songs became available. Smartphones allow different ringtones per contact, letting you identify the caller by sound alone.
Ringtone history is closely tied to mobile phone evolution. The late 1990s "chaku-melo" (ringtone melody) boom saw polyphonic ringtones played through built-in sound chips become wildly popular, with ringtone download services forming a market worth hundreds of billions of yen annually. In the 2000s, "chaku-uta" (ringtone songs) emerged, playing original recordings, replacing the melody market. Today, with streaming services widespread, paid ringtone downloads have declined, and pre-installed tones and custom settings are the norm.
Ringtone settings differ by OS. On iPhone, go to Settings > Sounds & Haptics > Ringtone, where you can also set songs purchased from iTunes Store or custom tones created in GarageBand. On Android, go to Settings > Sound & Vibration > Ringtone, where any audio file on the device can be set as a ringtone. Both OSes allow per-contact ringtone assignment through the contacts app.
Ringtones can also be used as a nuisance call countermeasure. Setting the ringtone to silent for unregistered numbers means you won't be disturbed by unknown callers. iPhone's "Silence Unknown Callers" feature automatically silences calls from numbers not in your contacts, sending them to voicemail. Combined with call blocking, this is highly effective. In public spaces, setting manner mode (vibration only) is standard phone etiquette. See Smartphone Privacy Settings for detailed call management.