Ever dial 111 from your mobile phone?
I did tonight, to report some street fighting. My phone went into "emergency mode", in which the menus are disabled, and the only number you can dial is 111. It disables the power button, and there's also an option to show your phone number without ending the call or going into a menu, which I'd imagine might be useful for reporting to emergency services should they hit a technical problem.
Emergency mode also uses any available signal that the phone can use, regardless of programming, or who operates the network.