If the HF is tuned to a radio station playing music, the SELCAL can be triggered. One airframe I used to fly alway had the SELCAL triggered by the music that preceeded the 'pips' on the BBC world service as the music onviously had two successive tones that were identical to the aircraft's call tones. The SELCAL would always trigger about 15 seconds to every whole hour if the world service was tuned.