Originally Posted By: Russ
I've wondered whether a small helium balloon on a long tether could take a cellphone (with a text message ready to Send) up to a point where cell coverage was available... Keeping a Spot or PLB in your pack would probably be easier.


It would depend on the local terrain and the cell phone protocol (CDMA vs GSM). Per the Wikipedia article on cell sites, the effective line-of-site range can drop down to 3 miles in the mountains.

Originally Posted By: https://en.wikipedia.org/wiki/Cell_site#Range
The maximum range of a mast (where it is not limited by interference with other masts nearby) depends on the same circumstances. Some technologies, such as GSM, normally have a fixed maximum range of 35 kilometres (22 mi), which is imposed by technical limitations. CDMA and IDEN have no built-in limit, but the limiting factor is the ability of a low-powered personal cell phone to transmit back to the mast. As a rough guide, based on a tall mast and flat terrain, it is possible to get between 50 to 70 km (30–45 miles). When the terrain is hilly, the maximum distance can vary from as little as 5 kilometres (3.1 mi) to 8 kilometres (5.0 mi) due to encroachment of intermediate objects into the wide center fresnel zone of the signal


About the only thing I can see reliably working is satellite (SPOT, PLB), and MAYBE one of the more powerful handy talkies.
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