喂(wèi) - hello (on the phone), hey; to feed in Mandarin

喂 (wèi) is the word you say when you pick up the phone. 喂，你好 (wèi, nǐ hǎo) is the standard opening, and on its own 喂 does the work of "hello?" when you are not yet sure anyone is there.

Two practical notes. The tone drifts: the dictionary reading is wèi, fourth tone, but on the phone most speakers answer with a rising wéi, and a long drawn-out wéi is the Mandarin equivalent of "hello? are you still there?". And 喂 shouted at a stranger in the street is not polite. It sits closer to "oi" than to "excuse me", so ask for directions with 请问 (qǐng wèn) and save 喂 for the moment somebody is about to walk into traffic.

Read wèi, the same character also means to feed, whether an animal or a person: 喂猫 (wèi māo), to feed the cat, and 喂药 (wèi yào), to give somebody their medicine. The mouth radical 口 explains both senses at once.