Wikipedia
Selcall
Selcall ( selective calling) is a type of squelch protocol used in radio communications systems, in which transmissions include a brief burst of sequential audio tones. Receivers that are set to respond to the transmitted tone sequence will open their squelch, while others will remain muted.
Selcall is a radio signalling protocol mainly in use in Europe, Asia, Australia and New Zealand, and continues to be incorporated in radio equipment marketed in those areas.