喂 (wèi) is an interjection used to answer the phone (like 'hello?') or to call someone's attention ('hey!'). Unlike 你好 (nǐ hǎo), which is a polite greeting for in-person or formal situations, 喂 is only for phone calls or getting someone's attention from a distance. 哎 (āi) and 嘿 (hēi) are more casual and used in face-to-face situations to express surprise or get attention, not for phone answers.