Final answer:
SELCAL systems enable air traffic control to alert an aircraft using VHF and HF radios. An incoming SELCAL call activates an audible alert, and some systems may indicate a 'C' on the MIC switch on the ACP, signaling pilots to tune into the communication frequency.
Step-by-step explanation:
The systems available for SELCAL (Selective Calling System) are generally found in aviation communications, and they enable air traffic control to alert an aircraft without the need for continuous listening on radio frequencies. SELCAL integrates with both VHF (Very High Frequency) and HF (High Frequency) radios on an aircraft. Recognition of an incoming SELCAL call usually triggers an audible chime, light, or other clear indication in the cockpit, informing the pilots that they have been called. If the question is referring to a particular SELCAL system model's operation, for instance, where a 'C' appears on the MIC switch on the Audio Control Panel (ACP), this would be specific to the aircraft's equipment and the SELCAL system design. Therefore, when a call is received, the pilots are notified and can then tune in to the appropriate radio frequency to receive further communication.