Our particular segment of the Allegheny Highlands Trail has a long stretch starting at Garrett with no cell service at all; there isn't much 'till you get to Connellsville. While a cell would be a huge help, in a crisis, it would take an hour or more before the state police could reach you. That's not to say that there shouldn't be more cell towers to serve the trail.
This would be an easier solution than building/maintaining fixed phones.
This trail in is very remote country, with few road crossings, and the trail follows the Casselman and Yock Rivers, often deep in a canyon; it is a beautiful trail.
For trails like this, you need a reliable bike, tools and spares, a first aid kit...and a gun. Metaphorically speaking, you may well have to kill your own snakes.