The best form of off-grid, trail communications is, by far, amateur ( also known as ham ) radio. Specifically, the 2 meter and 440MHz bands of the amateur radio spectrum are best rig to rig, on trail communications.
So, why isn't everyone using it? You need to be licensed to transmit on these frequencies and you need to learn how to use these radios.
Passing a test to obtain a license is not terribly hard, but depending on your existing knowledge, some study time will be needed.
The technician class license is all that's needed for typical trail communications.
Why is it better than CB or FRS ( the vacuum pack, handheld Walmart radios) ?
1) Less interference. Fewer people are using these frequencies and the ones that are using them follow a protocol to avoid unnecessary radio traffic.
2) Higher power. A CB runs at 4 watts. An FRS radio runs at less one watt. A typical 2 meter, mobile ham radio has a selectable power setting between 5 and 50 watts ( sometimes more ).
3) Better format that CB which give a much cleaner signal. FM vs AM. A CBs AM signal puts information (your voice) on the top and bottom of a radio wave going out of your antenna. As this radio wave travels through space noise is picked up and it's the top and bottom of the radio wave that pick up the most noise.
An FM signal puts information ( your voice ) in the middle of that wave which makes for a clearer signal ( voice ) on the other end.
4) Repeaters; Receive / transmit systems placed high on mountain tops that will extend your signal dramatically. This is where you will have get really familiar with how to use your radio. Repeaters are awesome, but depending on your radio, they can be overly complicated to program into your radio.
Basic settings to remember for repeaters are;
Frequency ( the receiving frequency / what you will listen to)
Offset frequency; You need to transmit into a repeater at a different frequency than you receive. Your offset is how many Hz (Hertz / cylces per second) your transmit frequency is offset from your receiver frequency. 2 meter repeaters are typically 600KHz = 0.6MHz offset. 70cm / 440MHz repeater standard offset is 5MHz
Offset direction; Which direction is that offset? Positive or negative /+ or -
Tone type and frequency; A transmit tone is also needed to get your signal through a repeater. This is an inaudible tone that is embedded into your voice signal. The tone acts as key to "unlock" the repeater.
Using the Big Bear K6BB repeater as an example, here is how you might see a repeaters information listed;
147.330 + 131.8
The 1st part, 147.330 is the listen frequency in MHz.
The next part, +, indicates the offset direction as positive. Now this is a 2M repeater and the offset is standard at 600KHz = 0.6MHz, so this means the transmit frequency is 147.330MHz + 0.6MHz = 147.930MHz.
The last part, 131.8 is the tone frequency needed to open the repeater.
You will have to dig through your radio menus to find and change each of these settings. Remember the typical offsets (600KHz for 2M, 5MHz for 70cm) and the 4 primary pieces needed; Frequency, offset direction, tone frequency and turn your tone on and you'll get it.
Not all radios are the same, so the terms used for these setting may be different from one radio to another. Tone may be "tone" on one radio, but "CTCS" (Continuous Tone-Coded Squelch) on another radio.
5) Additional features, beyond simple voice communications, such as APRS. It's like "find my friends" with no cell service... and there is even a way to send a text message from one radio to another or from a radio to a phone.