Depending on your class, they can be picky about actually displaying a protest flag and a hail is not valid, if you ever see a whiffle ball hanging on a dinghy boom, it's got the flag in there!
Depends on size of boat. Less than 6m flag is not required (usually).
As for hailing protest; that is not required either. If you foul, you should do your turn(s).
It's possible you get seen fouling, but out of earshot and so you cannot hear the protest hail. 
