Phones
GSM phones operate in 2 frequency bands in Europe - 900MHz & 1800MHz. In North America a 1900MHz band is used for GSM networks. And frequency hop, but not during a call.
GSM phones also modulate the amount of power used for transmission. This improves battery life and gives the base station a chance of handling signals from a few metres away to signals from kilometres away.
The base station aerial design makes most of the signal travel in a roughly horizontal direction. Which means at 30,000feet if a phone did find a base station, it would probably transmit at it's max power (only 8watts I believe for handsets). The phone, basestation and network would have no problem whatsoever in maintaining contact.
I predict within a few years, airlines will introduce phone access on aircraft. Probably not GSM, but maybe 3G networks if they work with manufacturers to certify the equipment. Allowing people to use their own phones on a long flight would be a nice little earner.
All the best
Mark