Wiretap Act, ECPA, and Stored Communications Act
The Wiretap Act (Title III) strictly bars intercepting calls; ECPA extends this to electronic communications. Federal law permits one-party consent, but many states require all-party consent, and federal law is NOT preemptive. The SCA governs stored communications with simpler exceptions.
The Wiretap Act (Title III) strictly prohibits intercepting wire (aural) and oral communications; Electronic Communications Privacy Act (ECPA) extends the ban to electronic communications like email. Real-time interception is treated more strictly than retrieval of stored records.
Federal law permits interception if one party consents. Many states require all parties to consent (hence "this call may be recorded"). Federal law is NOT preemptive, so an organization recording calls can still violate a stricter all-party-consent state law. Video conferencing may fall under these state laws too.
The Ordinary course of business exception permits interception with provider-furnished equipment in the normal course of the user's business - e.g. routine call-center monitoring or scanning company email for malware - but listening to a purely personal call risks violating the wiretap laws. Courts split on how broadly to read it, so many employers rely on consent instead.
The Stored Communications Act (SCA) bars unauthorized acquisition, alteration, or blocking of communications in electronic storage. Its exceptions are simpler: conduct authorized by the entity providing the service (often the company) and by a user of the service. ECPA does not preempt stricter state law - e.g. Delaware and Connecticut require employee notice before electronic monitoring.