Sorry, just some more clarifying questions to help someone answer your question.
Knock yourself out.
I assume that you're talking about Israel, right? How popular are mobile phone carriers that use iDEN in Israel?
Yes, Israel. There's only one company. Virtually all cab drivers, paramedics, firemen, policemen and military ranks seem to have one. It's IDF's official phone, I believe. Private ownership is scarce.
When you say "mountains," I assume that you mean away from any cities/towns, like during hiking? And that probably also means away from most radio towers. That's going to be a challenge for any technology, if that's what you're thinking of.
Well, it's difficult to get far away from anything around here - we
are talking about Israel. No, I'm more concerned about the highland nature of local geography.