US Electronic Communications Privacy Act, Title I (Wiretap Act, 18 USC 2510 to 2523)

Unless the chapter specifically provides otherwise, no person may intentionally intercept, try to intercept, or procure anyone else to intercept or try to intercept, any wire, oral or electronic communication. For an employer this covers real-time capture of calls, email or messages in transit, keystroke or screen tools that acquire message contents as they are sent, and listening devices, unless an exception (provider, business extension, consent) applies.

A person not acting under color of law may intercept a wire, oral or electronic communication where that person is a party to it or one of the parties has given prior consent, unless the interception is for the purpose of committing a criminal or tortious act against the Constitution or the laws of the United States or any State. Employers commonly obtain the employee's prior consent through a signed monitoring policy; consent must cover the monitoring actually carried out, and state all-party consent laws can still apply.

The prohibition turns on the use of an electronic, mechanical or other device. Telephone or telegraph equipment or facilities, or their components, furnished to the subscriber or user by a provider in the ordinary course of its business and used by the subscriber or user in the ordinary course of its business (or furnished by the subscriber or user to connect to the service and so used), and equipment used by a provider in the ordinary course of its business, are not such a device. An employer's use of its phone system to monitor business calls can fall outside the prohibition where the monitoring is in the ordinary course of business; hearing aids are also excluded.