Example:
Aircraft:" SFO radio, SFO radio, this is cessna 123 on 8 decimal 9, over".
(lots of static and other pilots talking)
ATC:"Cessna 123, SFO radio, go ahead".
Aircraft:"SFO radio, cessna 123 request SELCAL check, AB QM, over"
ATC: now keying up on the four letters, sound of musical chime heard (ABQM).
If your system works properly, you will get the alert on your radio, enabling you to not actively monitor this noisey frequency.
Aircraft: "SFO radio, cessna 123, SELCAL check ok, out"