Complex sentence: He called "Mayday" five times, but because the radio was still tuned to Larnaca, not Athens, he was not heard by ATC; his voice was recognized by colleagues who listened to the CVR recording.
Split into simpler sentences: He called "Mayday" five times but, because the radio was still tuned to Larnaca, not Athens, he was not heard by ATC. His voice was recognized by colleagues who listened to the CVR recording.