4 ideas
Maggie Byrnes, a young operator on the same staff, narrates the case and runs her own investigation alongside the police, working through her colleagues' shifts, grudges and secrets until more violence follows. The author had worked as a telephonist herself, and the book draws on the exchange's real routines of plug boards, rostered shifts and rest rooms.
A switchboard lets operators hear private conversations, so anyone who listens in builds up knowledge that can be used for leverage or blackmail. The murder weapon is itself a listening device, which ties the crime to the fact that overheard information is dangerous to the listener as well as to the people overheard. Once you see this, you ask who could hear what before you ask who hated whom.
In a closed workplace with fixed shifts, sign-on times and controlled spaces, the organisation's own procedures decide who had access to the victim and when. Solving the crime becomes a matter of reconstructing the roster, the break schedule and who moved between switchboard and back rooms. The bureaucracy produces the alibis and also holds the gaps in them.
An operator knows the unwritten social rules of an all-female shift workforce, including rivalries, loyalties and who covers for whom. She can read clues that police treat as noise. Because she has little formal authority but deep tacit knowledge of the workplace, she can put together a picture the official investigation cannot reach from outside.