Many aircraft also have satellite phones that controllers can call.

WSJ: How New Air-Traffic Control Systems Operate Over Oceans

Low cost airlines like easyJet, Ryanair and Go are buying new aircraft and adding new flights faster than controllers can be found to handle them.

BBC: The future of flying

Radar, or radio waves, sweep the sky every 12 seconds transmitting images of approaching aircraft that can then be viewed by controllers onscreen in a series of centers spanning the nation.

FORBES: Magazine Article