Of course it is proper to use 喂 outside of a phone conversation.
1. If someone is impolite to you or say some dirty words to you , you can respond : 喂， watch your behavoir or watch your mouth.
2.Mum feeds her baby : the young lady is 喂 her baby with milk.
3. When you think something is outrageous , you can say : 喂，this is too outrageous .
and bla bla bla ...................
anyway , 喂 is a model particle in most of conversations , but it can be a Verb , too ，such as 喂 baby .
hope this can help you understand it .