Well, you can use "was?" to ask "excuse me?", "pardon?", "say again?" etc. In this case, it is more polite to ask "wie bitte?" instead of "was?". Other than that, "was?" is basically the same question word as "what?" in English and in all other meanings of the word you cannot replace it with "wie bitte?". Imagine the following dialogue in German:
A: Guess what I just saw.
If B asks "was?" (with a falling tone) he says that he wants to know the answer ("was?" = "was hast du gesehen?").
If he asks "wie bitte?" (or "was?" with a rising tone) instead he implies that he didn't understand A's question and wants him to repeat it ("was?" = "was hast du gerade gesagt?", "what did you just say/ask?").
(I'm sorry if this is obvious, but the information that you can replace "was" with "wie bitte" is misleading IMO).
When waiting in Germany at a bus stop--I knew little German at the time--two boys, one young and the other younger, began directing questions at me, though my back was to them. I did not realize they were addressing me. Then a lady there politely (in Germany, that means gingerly and pleasantly, as it does elsewhere) came over to tell me they were asking me what the English question "What's up?" means in America. The boys had been arguing about it, the older saying it meant "What is happening?" and the younger insisting it meant "What is [physically] up?"
I answered in slow English for them, telling them that I was an American and did not speak much German, but that "What's up?" indeed means something like "What is going on?" Shocked at their having found a real American to answer their question, the older boy loudly exclaimed--I will never forget it--"WAS??"--their mouths dropping bang slap wide open. And the younger boy tore off and ran away as fast as any boy ever did. He came back a little later, and the boys had found their answer.
No, it was not the polite "Wie, bitte?" discussed in the other comments on this page. In an emergency, exclamations just happen.