喂
wéi
hello (when answering the phone)
This is an interjection or onomatopoeia — it represents a sound.
喂，你好，请问是张先生吗？
wéi, nǐ hǎo, qǐng wèn shì zhāng xiān sheng ma？
Hello, hi there, may I ask if this is Mr. Zhang?
电话响了，她拿起来说了一声喂。
diàn huà xiǎng le, tā ná qǐ lái shuō le yì shēng wéi。
The phone rang, and she picked it up and said hello.