One of the tasks you need to consider if you get lost or are injured and can not reach the trailhead, is a way to get attention of rescuers. Depending on the environment where you are stuck, you can signal for help in many different ways. It's a good idea to set up a few different ways, if you can.
Be sure to set up your signals in the most visible place available to you so they can be seen from as many directions as possible.

Call For Help

In some frontcountry hiking areas, cell phone reception is pretty good. The further from civilization you go, the less helpful a cell phone becomes.
In the areas I hike, they seldom work so I've never taken one along.
If you don't mind the extra ounces of weight and space used up in your pack, then it won't hurt to have one. It would sure be courteous to the rest of us if you left it turned off and only really used it for emergencies. I can't think of anything more annoying than hearing a ringtone down the trail while I'm soaking up some sun on a rest break along the trail.
Another one that irked me last summer was being on top of an amazing mountain and having to listen to people calling home to say, "Hey, I'm on top of Mt. Whatever and it's amazing! Yadda, Yadda, Yadda, Blah, Blah, Blah, ..."
 
There are many other electronic devices available that you should consider, especially if you are thinking of solo hiking:

• Cellphone - while hiking close to populated areas or travel ways, your cellphone will probably have coverage. Even out in the wilds, you may get a signal. On wide open, flatlands the cell coverage is better. Hike to the top of a ridge and your chances go up also.
• Personal Satellite Devices - one-way or two-way devices that send (and possibly receive) text messages, emails, and tracking pings. There are new ones coming out annually, but some brands are SPOT Messenger, Garmin InReach, Zoleo, SomeWear.
• Satellite Phone - pretty much guaranteed two-way communication any place, any time. You can rent one for specific trips rather than purchase.
• Personal Locator Beacon - simple one-way satellite device. Activate when you want to be rescued and then wait for the calvary to show up.
• Cellphone Satellite Support - as cellphone coverage builds, we will soon have global coverage and all these other devices will be dinosaurs. Cellphones using satellites for more than just GPS pings will allow easy wilderness communication.

Rule of Three

In the United States, three of something signifies that help is needed. That could be three whistle blasts, three gun shots, three piles of dark wood on light sand, or three fires burning. The fires can be in a line or a triangle, but make sure they are pretty far apart and in an open area to be seen from overhead.
If you are using a whistle, blow three distinct times and then wait 3 to 5 minutes and blow three blasts again. Continue to do this every 15 minutes or so.

Signal Mirror

Reflecting the sun's light to distant places, such as an airplane or helicopter, can be very effective at getting attention. A mirror can reflect light miles across the open. If you can get to a high spot, you can spend time signaling to distant places on the horizon until you see an airplane flying. It's better to signal at a plane that is in the distance rather than one that is up above you since the pilot can't see straight down.

There are specially made signaling mirrors with a hole in the center for sighting. But, any shiny surface can work - a compass, watch, knife blade, ... are all possibilities.
Sight the reflection on the distant target and keep signaling it until you get a response, such as the plane dipping its wings or the helicopter circling overhead.

Smoke

Once you have your survival fire going, gather more wood into an open area and lay a large fire ready to be lit. Also, gather dry leaves, pine needles, moss, green pine boughs, and a bottle of water. This will be your signal fire and you want it to be pretty big and as smoky as possible.
When you have enough wood and material to create smoke, light the fire. You may want to wait until morning if it is already dark for the evening - chances of a search party coming out at first light is greater. On windy days, smoke will dissipate quickly so early morning tends to be less windy than evening, too.
Get your fire burning and then add smoke materials to create a large column of smoke. As you experiment, you can figure out how much material to add to create smoke but not smother the fire too much. Creating a large puff of smoke periodically rather than burning all your materials right away may increase your chances of being seen.