Yes when can "see" a transmitter with enough signal strength
Cell phones just need to be able to get a strong enough signal. That is why they "come to life" as the aircraft descends, but might even be OK very high up. The problem (for the cell network) is that when a phone is high up it may "see" many more cells than would be normal and give the network a problem (so I am told). That is why they are banned in flight, nothing to do with safety (except during landing and takeoff, very wisely!). See recent article in "The Economist".
However, all this will fail for commercial reasons, just like ordinary phones in the aircraft and also in flight internet access, recently withdrawn by Boeing. See my other post