The term SIGNAL TO LEAVE A MESSAGE can be broken down into two crossword solutions, BEEP and TONE. BEEP is a short, sharp sound made by an electronic device, such as a horn or a telephone. It is often used to indicate the start or end of a message. TONE is a sound that is produced by a musical instrument or other sound-producing device. It is often used to convey a particular emotion or feeling. Both BEEP and TONE are commonly used as signals to leave a message.