Simple ATC provides for a more immersive ATC communications experience by using the text to speech capabilities of the operating system to synthesize communication between your aircraft and ATC. This capability is built into X-Plane but the voices tend to be very robotic and it does not handle the various flight plans such as VFR and VFR with flight following.
Simple ATC is not intended to be a full blown ATC add-on. For that we would recommend either Pilot2ATC, 124th ATC, or flying on virtual networks like Pilot Edge, POSCON, and Vatsim.
Simple ATC is designed to work best with the Microsoft SAPI voices. It will work using the one X-Plane voice on OSX but it is not as immersive as having multiple SAPI voices on Windows.
