Selcal is Selective calling.
So that aircrew do not have to maintain a listening watch on HF radio and listen to the incessant backgroung noise selcal is used. Each aircraft has a unique selcal code on receiption of the code a chime sounds and usually a light comes on to alert the crew that they are being called. The code as transmitted consists of two pairs of tones hence the Beee Bahhh. On a protracted listen to the North Atlantic Net you will hear aircraft, on their initial contact with Gander, Shanwick etc. give their position, track information and their selcal code in the form "selcal is abcd" The ground station will transmit the code for a check and then the aircraft will maintain selcal watch just listening to HF when giving position reports. ok?