SquawkBox 

SquawkBox is an add-on for Microsoft Flight Simulator. It is a simple and unobtrusive application which provides the core functionality needed to get you connected to online air traffic control networks. It adds tremendous realism to your flights by connecting you to an online world populated with virtual air traffic controllers. These controllers are real people that communicate with you on voice, handling every aspect of your flight: filing your flight plan, getting clearance, pushback, taxiing, takeoff and landing.

SquawkBox is 100% free to download and use.