D.I. Whiteman is talking to his unknown female 'contact' from a phone box. When she hangs up on him, he hears the dial tone. But the dial tone used here is the modern, digital dial tone, which was not used until 1970 or so. In any event, when the other party hangs up first, there is only dead air, no dial tone, but dubbing in the tone is an oft-used show-biz effect.