I take it the hacker cannot listen in on an actual conversation, like a wiretapper, but can get to the subject's voice-mail system and play back the messages.
From the sfgate.com
A wiretapper could even choose to target the phone calls of a company's general counsel talking to an outside law firm, or the CEO talking to his counterpart at another company.
From the forbes.com
With the help of his pregnant wife, Connie, O'Mara studies the files of potential squad members and puts together a team of tough guys, plus one expert wiretapper.