SelCal is the acronym for "selective calling". Because it is virtually impossible to "squelch" the HF receiver of noise, static and undesired communications, a "listening watch" for calls from ATC would add significantly to cockpit workload and distraction. SelCal is a tone activated squelch system that keeps the receiver muted until the proper tone code is received. It is actually two pairs of tones, similar to telephone "touch tones". When a controller wants to contact a particular aircraft he transmits the tones, the receiver decodes them, unlocks the audio and the controller as well as any other transmissions can then be heard. In addition there is usually a chime indication so that even if the pilots were listening in the background they are alerted when a call comes for them. If their SelCal system is not working properly they are required to continuously listen, which is why you will hear them getting a SelCal check prior to entering Oceanic airspace.