Thus, crewmembers need not devote their attention to continuous radio listening.
HF radio often has extremely high levels of background noise and can be difficult or distracting to listen to for long periods of time.
The crew next turns up the volume on the aircraft radio to hear the incoming voice transmission.
Using ICAO radio protocol, they must verify with the transmitting operator that they are the intended message recipients.
The letters within a given pair are written or transmitted in alphabetical order (e.g., AB-CD is an allowable distinct SELCAL code, as is CD-AB, but CD-BA is not).
A SELCAL 32 Expansion Project is started after October 2018 under evaluation to increase the number of possible allowable codes.