The handsets are always making search transmissions and at full power. When they can't get a response they ramp up the power and try again. At high power outputs the transmission power is sufficient to break through on my tape player.
Is the above for a GSM network? Surely not.
AFAIK GSM phones remain silent until they pick up a signal from a tower. Then and only then they run the protocol with that, at a power which is adjusted according to how strong the incoming is (stronger means less power is required from the phone).
I still don't believe there is a significant chance of a mobile phone interfering with anything.
Which is a different thing from banning them as a precaution....