I thought I had seen this question on this forum, but can't find it now. If you have your BB device with you while you are somplace without a phone signal, can Emergency Medical Services find your location by your GPS signal?
You don't need a carrier to use GPS. If your GPS can figure out where you are (with Google maps or so) in a non-signal phone area, others (EMS?) can figure it out too. I believe however that you need some kind of subscription somewhere to make you "findable".