Off-topic, but: in the days before Gogo was installed on all AS's aircraft, I flew ANC-DEN and accidentally left my iPhone on without Airplane Mode enabled. I heard a sound from my phone about two hours into the flight and pulled it out of the seat pocket to find a Facebook push notification. I was slightly perplexed for a brief moment until I saw one bar of service, a 3G logo, and the word "TELUS" in the upper-left corner.

I also got a bill for $10 in roaming charges later that month.

It probably wouldn't have been a strong-enough signal to carry on a conversation, but cell phones can occasionally latch onto a signal from the ground. There's not a lot of interference to attenuate the signal between the sky and the ground.