Title III of the Omnibus Crime Control and Safe Streets Act of 1968 defined the guidelines for intercepting wire and oral communication. Unless the employer is monitoring the phone call for quality control (i.e., technical support for customers), the employer may monitor work calls without notice. However if the employer learns that the phone call is about a personal matter, the employer must not continue to monitor the call upon learning that a personal matter is being discussed.
The Electronic Communications Privacy Act of 1986 amended Title III to include electronic communications. See Pub. L. No. 99-508, 100 Stat. 1848 (codified in sections of 18 U.S.C. including Sec. 2510-21, 2701-10, 3121-26). More importantly, intercepting these communications (e.g., email messages) can be based on suspected violations of any Federal felony. See 18 U.S.C Sec. 2516(3).