You can monitor your employees' communications, within reasonable limits. But you can't let your monitoring escalate to eavesdropping and violate workers' privacy rights.
It's perfectly legal, for example, to listen to employee phone calls with clients or customers for quality-control reasons. Some states require you to inform the outside party that someone is listening.
Federal law lets employers monitor employees' work calls unannounced. But the law makes an exception for personal calls. Once a supervisor realizes that a call is personal, he must immediately stop monitoring the call. It's best to notify employees of your monitoring policy and obtain their acknowledgment in writing.
Recent case: When a police officer called his wife from his office, he voiced approval for a department restructuring plan that would hurt some of his superiors. His wife responded that the "SOBs [would] finally get what they deserved."
The police department automatically recorded all calls on certain phone lines. Later, the officer learned that the conversation had been recorded, and the assistant police chief had played it for two other co-workers. The officer sued, alleging privacy violations by the secret recording of his conversation (Fourth Amendment), and a Louisiana jury agreed. (Zaffuto v City of Hammond, No. 308 F.3d 485, 5th Cir., 2003