Cell coverage, or rather lack thereof, was a problem for me in my effort to go high altitude camping.
Satellite Communicator from Delorme to the rescue.
http://www.smallrvs.ca/?p=454
While not a PLB, it will also do great to alert family to contact CAA/AAA and if no-one replies you can use the SOS function to send a message to a 24x7 facility.
It made for a bit of extra peace of mind while travelling from Baker, NV to Yosemite via Highway 6. A very interesting but definitely lonely stretch of road.
Heinz