The SELCAL is used in real flights to prevent that the pilot passes long hours listening to the ATC, expecially in HF frequencies, where the static noise is very strong. In case that the controller needs to speak with the pilot, he will send the SELCAL code of the airplane, making to sound in cockpit a sonorous and/or luminous alarm, giving indication to the pilot of whom its presence to the radio is necessary. Each airplane has its own pre-defined SELCAL, never being able to be modified. The Selcal works with the sending of 2 groups of 2 tones, each one different one of the other, chosen of a pre-defined stack of frequencies inside of the audible specter. The used frequencies are not harmonics, thus reducing interferences. All pilots in the FIR listen to the SELCAL but when the appropriated SELCAL for the plane is played a second warning will sound in the cabin.
FSSelcal implements this kind of facility in the virtual flight. This will allow pilots/controllers to get a more accurate real life rendition. It does not substitute the 1/2 hour VATSIM unintended flight rule.
Please visit FS SELCAL homepage for more information about SELCAL procedures at http://www.satavirtual.org/fsselcal/