Several of the SAR reports note that victims were able to get a 911 call through. However, cell reception is VERY difficult to get. [1] [2]

cell tower location

Individual towers:

how to build a parabolic reflector

Designs for a folding parabolic reflector. (hopefully one that will fit in your backpack without too much trouble) This is inspired from the "Poor Man's WiFi". Parabolic reflectors are widely used.