Why is 喂 used over 你好 when answering the phone?
Most languages use an alternate greeting for telephone calls; the English &quot;hello&quot;, although originating from before the telephone, was popularised by it, so much so that it has become a common greeting outside the telephone:

hello
1883, alt. of hallo (1840), itself an alt. of holla, hollo, a shout to attract attention, first recorded 1588. Perhaps from holla! &quot;stop, cease.&quot; Popularity as a greeting coincides with use of the telephone, where it won out over Alexander Graham Bell's suggestion, ahoy. Central telephone exchange operators
were known as hello-girls (1889).

So the mandarin 喂 is not an exception, only that unlike &quot;hello&quot;, people do not use it outside telephone calls.
The reason for telephone greetings is that it was a medium where reception may be bad and you are unsure who you are talking to. Thus a short greeting meant to solicit response (i.e. &quot;can you hear me&quot;) is used.