Sentence:  Six radiomen told how , twice on two days after the ring was nabbed , a transmitter near Moscow was <WSD>heard</WSD> calling , using signals , times and wavelengths specified on codes found hidden in cigaret lighters in Lonsdale 's apartment and the Krogers ' house and also fastened to the transmitter lid . 
Meaning of the word marked with <WSD> tags:  perceive (sound) via the auditory sense