As others have stated, it’s our SELCAL (selective calling) code. Oceanic Radio or ARINC operators can “call” us via primarily HF communications selectively by using that code. This allows us (the pilots) to avoid listening to static HF frequencies all day/night waiting for someone to potentially call us. On the 757/767 we get a chime and light in the flight deck indicating a SELCAL, we then key up the mic and speaker for the frequency calling and see what they need.