Sentence:  Six radiomen told how , twice on two days after the ring was nabbed , a transmitter near Moscow was heard <WSD>calling</WSD> , 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:  send a message or attempt to reach someone by radio, phone, etc.; make a signal to in order to transmit a message