who "were dependent upon the cooperation of.." Smith &Bell, or "fingding another line?"
Smith and Bell shared the same desperate plight. They weren't learning much where they were. Yet should either surrender his outside line, the chances of finding another were negligible. They were dependent upon the cooperation of colleagues and tolerant passers-by who, hopefully, would be reliable. It is a fact, and it is something of a miracle, that the megaphone worked.