Hush-A-Phone v. United States, 238 F.2d 266 (D.C. Cir. 1956) was a seminal ruling in United States telecommunications decided by the D.C. Circuit Court of Appeals. Hush-A-Phone Corporation marketed a small, cup-like device which mounted on the speaking party's microphone, reducing the risk of conversations being overheard and increasing sound fidelity for the listening party. At the time, AT&T had a near-monopoly on America's phone system, even controlling the equipment attached to its network. In this era, Americans had to lease equipment from "Ma Bell" or use approved devices. At this time Hush-A-Phone had been around for 20 years without any issues. However, when an AT&T lawyer saw one in a store window, the company decided to sue on the grounds that anything attached to a phone could damage their network.

AT&T, citing the Communications Act of 1934, which stated in part that the company had the right to make changes and dictate "the classifications, practices, and regulations affecting such charges," claimed the right to "forbid attachment to the telephone of any device 'not furnished by the telephone company.'"

Initially, the Federal Communications Commission (FCC) ruled in AT&T's favor. It found that the device was a "foreign attachment" subject to AT&T control and that unrestricted use of the device could, in the commission's opinion, result in a general deterioration of the quality of telephone service.