A cop in America radios for backup in a forest, where a gunshot's just been heard. Would be use somekind of callsign or something when radioing back? I should know this but my mind's gone blank.
It will depend on the level: Federal, State, County. I guessing your example would most likely be a county Sheriff's office of Local Police (namely a small department). Call signs as far as I know vary from one place to another (we all know the good old "104 good buddy" saying) but I know that that call is used in some counties and not in others. It varies. Call signs like numbers are usually only used in larger cities where the abundance of crime dictates speedy communication but in the woods I would think "This is deputy Joe, we have shots fired in the woods just of Lakeview Drive" and then the other responding "Backup is on the way" or the like would suffice.
i think loh has nailed it pretty well... this is not a normal city 'on duty' situation, so just the simple kind of call for help and response he suggests is most likely, imo...
Why is it off-duty? Rural cops have to deal with back roads and woods, especially if they're out there looking for someone, and he wouldn't have his police radio with him if he wasn't on duty; it'd have to be a personal CB set that for some reason he was lugging around the woods...
it doesn't have to be... but for a 'cop' to be in a 'forest' it sure sounds like that, doesn't it?... plus, in rural areas, which a 'forest' certainly would be, it's much more likely that law enforcement will be handled by deputy sheriffs or the forestry service and not 'cops'... however, all that is moot, since the op hasn't given us much info on the situation...