Over on YouTube content creator Tech Minds continues to upload informative RTL-SDR based videos, this time discussing SELCAL and the HF Air Band. For international flights it is common for aircraft to communicate with ground controllers and the parent company via the HF bands.
As radio communications are sparse, and the pilots obviously don't want to monitor noisy HF static for the entirely journey a system is required for signalling pilots when a ground station wishes to communicate with them. The system in use today is SELCAL which simply consists of transmitting a set of tones unique to an aircraft. When a correct SELCAL tone is received the aircraft system alerts the pilots that a radio voice communication is about to come through, allowing them time to get the radio in operation.
Tech Minds' video explains this in a bit more detail, and shows some examples of HF air comms with SELCAL tones played.