喂 is not only used in phone call, it was even used before the invention of phone. If you have watched lots of Chinese movies you may find that before people yell to another guy in mountain across a valley, yell through a tunnel, a pipe or even a well also start with 喂, the word 喂 is usually used as attracting attention and building connection.
After we can use a phone to talk to a people far away, we also consider this thing is similar to a pipe, tunnel or talk between two mountains, we need to build connection first, so we say 喂 to attract attention. Today, phones are very popular and people are used to start with 喂, using other words as greeting for phone calls make people uncomfortable instead.
再见 should not be translated as "see you again", the meaning of 再见 is just good bye, many Chinese words' meanings are not just the combination of the meanings of the two characters, you should be careful with that in your further learning.