I carry a antler made whistle in my pouch and I also have a metal one.
I had the metal one on my keyring before but I read that you are not allowed them on planes so I had to take it off. Not sure whether this was true or not.
My big rucksack also has an emergency whistle....not recommended to blow it in the house to test it.
If you are lost and do you 6 blasts, then you hear the 3 from the rescuers....do you just repeat until they find you or once they have replied do you wait for a period before doing it again?