Certainly one of the limitations to any version of Flight Simulator, including FSX, has been the inability to actually talk to the integrated air traffic controllers. Keyboard commands have been the only way for you to acknowledge or make requests of controllers. Bob Sidwick of RC Simulations says he has a product that changes all of that.
According to Sidwick, VoxATC is now available for FSX and, like it's FS9 predecessor, VoxATC X allows users to utilize headsets and push-to-talk buttons to communicate with Flight Sim's ATC by voice commands. Sidwick said both downloadable versions are available for trial at the VoxATC website and you may order both from the RC Simulations website.
