Its actually 'SELCAL'. Can't remember what it stands for. But basically its a system used in the Atlantic to call Aircraft that are on HF frequencies. As part of their Oceanic clearance to cross the ocean, they recieve a four letter code which is input to the aircrafts comms system. During the flight, pilots usually turn down the volume on their radios because of a few things...one being the noise from HF, the other being the fact that all aircraft entering the ocean enter separated and very little "controlling" is done until they are within radar range again. They do give position checks as they go along. Here in Ireland the HF station is called Ballygireen. Basically, these radio officers are in contact with aircraft for approximately half way across the Atlantic until they are transferred to Gander Oceanic (usually for A/C heading to the USA from Europe). Shanwick Centre, based in Prestwick in Scotland control aircraft on the North Atlantic. They relay messages to the aircraft via Ballygireen radio station. If Ballygireen want to call an aircraft they hit their SELCAL code and the pilots get an audio alarm alerting them to contact them by HF. So I hope this helps....