In a survey of 906 large firms by the American Management Association, 35 percent said they monitor their workers by recording their phone calls and voice mail, inspecting their computer files or even videotaping them on the job. And many of these companies don’t tell their employees that they’re being watched. Financial employers such as banks, brokerage houses and real estate offices are most likely to snoop; manufacturers are least likely. The most popular form of oversight: monitoring phone calls. About 37 percent of companies track the phone numbers called by employees and the amount of time they spend on the phone.