If you don't want to disturb the person you want to contact, the magic of ringless voicemail can be incredibly helpful. Every operator has a different answer.
Think about being in a meeting, relaxing, or otherwise preoccupied with something essential. And then an unexpected call rings or vibrates noisily on your phone. Users can leave voicemails with operators without disturbing the intended receivers. The people who will thank you are your friends, family, and coworkers.