On which U.S. TV espionage series would agents communicate with headquarters using the message, "Open Channel D"?

"The Man From U.N.C.L.E." was a NBC show that had everything for this genre of television. There was plenty of action, intrigue, and appropriate humor. The active agents in the series had a range of useful spy equipment. This included handheld satellite communicators. A catchphrase often heard was "Open Channel D" when the agents used their pocket radios. These devices were originally disguised as cigarette packs, later as cigarette cases, and still later as fountain pens.

In this cult 1960s classic television series, "The Man From U.N.C.L.E.", it starred Robert Vaughn as the quick-witted secret agent Napoleon Solo and David McCallum as Illya Kuryakin, Solo’s impeccably well-mannered, fairly no-nonsense fellow secret agent. Both men work for U.N.C.L.E. (an acronym for "United Network Command for Law and Enforcement"). They operated under the refined leadership of Mr. Alexander Waverly. They often worked smartly to get themselves in and out of terrible spy situations.