I spent a summer with two brothers from the Dominican Republic and they would call out to each other with a loud howl-like "FOOOOOOOOOOOO" sound that carried very well through fields, forests, and over a lake.
Later in life I spent some time with a friend of a friend who was essentially squatting and setting up a semi-permanent settlement in the jungle. Him and his gang of goons used the same sound, a howl-like "FOOOOOO" to signal their presence to each other in a remote Latin American 'city' on a hillside, and they said they used it in the jungle too. I was surprised to actually hear this and see it work for them to regroup from different parts of the city - it really carries well!
The folks from the latter story told me it's the sound that humans can make the loudest for the longest amount of time (key thing there is it's not only loud and carries well, it can also be drawn out for a long period). When you get the chance, try it out, and notice how long you can make it with relative ease (it doesn't take your breath away as other similarly-loud calls might).
I put it to practice and found it works. Myself and two others were relying on radios during work in the woods once and our team didn't meet as expected. The "FOOO" sound actually worked better than a pea-less whistle and the non-functional (due to terrain) radio I had, according to my team which heard me and eventually got back together.
Cup your hands by your mouth like a megaphone and howl out to the sky, "FOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!"
As for getting in trouble, generally the signal for emergency is three repeated calls. So, don't do that. If you just call once or twice and wait a reasonably long time before calling again, people who are listening are more likely to say "Oh, they're OK or they'd be SOS'ing or calling out more chaotically in a panic." The FOO sound specifically carries very well but doesn't have the sense of urgency that a high-pitched scream or whistle does, at least in my experience.