It will need to be loud to, hopefully, be heard at a distance.

In the woods and on the water, a whistle means distress.

There is the Orion Safety Air Horn Mini 1.5 oz. I have not heard the sound. If it sounds like an air horn, well, in the mountains people may assume "you" are playing around.

In fact, three whistle blasts, three blinks of a light, three flashes of a real marine signal mirror, with a sighting opening, are the universal distress signal. Three flashes of a shard of a mirror, if seen, is also a universal distress signal. The sighting mirror only allows better aim. Fire a gun three times, if the three shots are heard, people nearby will get in their truck and drive up in the mountains. No one really expects an SOS signal. Three is the universal distress signal.

It is also possible to carry a large blaze orange plastic sheet, presumably to put on the ground. If there is any chance of small airplanes up or helicopters in the air, this is a worthy signal. In some places, small airplanes and helicopters are routinely used for search. This is the best place for this type of signal.

I like orange smoke. It is not likely people would call 911. However, people might call 911. Forest fires have had that effect on people noticing any smoke. I would never however use white smoke. This is a crime. The fine is enormous. My bet is on orange smoke.

There is now land orange smoke made available to the public called Orion Orange Smoke Signal. Nevertheless, do not put it on something that could burn.

Any and all signaling is a bet.

People tend to pay no attention to universal distress signals.

I have been an advocate of PLB's for land use, restricted in the past to Alaska only.

I have advocated their use in "the lower 48" for many years.

The PLB Personal Locator Beacon is now no longer limited to Alaska or on the ocean.

The small-sized McMurdo Fast Find 210 (PLB) is available for land use.

The SPOT Satellite Messenger was the first in "the lower 48" for hikers and has a stronger gps receiver, than before.

Either one, if abused, could result in big fines. It is not a toy.

Because these PLB personal locator beacon technology have, on the whole, not been abused, there are new online services like Spot Connect.

Here is their video.

I do carry signaling whistle, signaling mirror, and orange smoke canisters because someone may help, and, these time-proved signals can help rescuers pin-point your position.

A loud whistle is better than shouting, and becoming exhausted.

For a whistle, I like Storm Whistle. It is so loud it is not to be used in a tent, or, indoors.

To get the most out of a whistle, use your chest and "huff". Reportedly, this is both the loudest and the least fatiguing method. Make three blasts, at intervals.

I would like to carry a satellite telephone like this one. Here is their rental page.