As someone who has been involved in SAR for years and has spent the last 40 years prowling the most remote areas of the US, I've found the only truly reliable communications from a lot of those areas is a satellite phone.
In Nevada, even Interstate 80 has areas with no cell phone coverage. Secondary roads even less, and away from pavement or in a canyon there is zero coverage. I'll qualify that by saying that if you can make your way up high onto a mountain there is a good chance of picking up a signal. I've been surprised at the locations I could make a call if I was at 10,000 feet or above.
2 meter - HAM - even the radios we used for SAR (Sheriff's Department) only work to a limited extent. If out of range/out of sight of a repeater they were useless. None of them could get a signal out of a deep canyon. During a search SOP was to set up a vehicle on the highest accessible mountain in the search area and use it as a relay. As a side note, when the county radios started migrating away from high band to digital (as mandated by the feds) communications efficiency deteriorated. Digital is nowhere as good as high band when long distances are involved.
I still have my SPOT but no longer use it because it too didn't work very well when surrounded by canyon walls. If there were no good view of the sky in a southerly direction the SPOT couldn't get a signal out. So I decided to try a satellite phone and purchased an Iridium 9555 unit. I pack it along with a spare battery. It has never failed to acquire a satellite. In a deep narrow canyon sometimes you have to wait until the next satellite comes over - and you have to be quick because when that satellite disappears beyond the canyon wall the signal disappears if the next satellite is out of sight.
And about that far to the next cell tower.......
