From Checkpoint Charlie: The Cold War, The Berlin Wall, and the Most Dangerous Place On Earth by Iain MacGregor (Scribner, 2019), Kindle pp. 64-65:
Checkpoint Charlie would be formally distinguished by the Four Powers as the single crossing point (either by foot or by motor vehicle) for foreigners and members of the Allied forces. Members of the Allied forces were subsequently not allowed to use the other sector crossing point designated for use by foreigners at the Friedrichstraße railway station. The name “Charlie,” though it would become quite catchy to fans of spy novels and films over the years, had a more prosaic backstory. The Allied checkpoints covering entry into East Germany, and then into Berlin, derived their names, simply, from the letters in the NATO phonetic alphabet. The Allied checkpoints on the Autobahn linking the city to the West were Checkpoint Alpha at Helmstedt/Marienborn and Checkpoint Bravo, its counterpart at Dreilinden/Drewitz in the southwest corner of Berlin. Soldiers of the US Army’s 287th Military Police Company would man this new crossing in shifts around the clock beginning on August 23. This small unit was then formally expanded, and a desk was placed in a nearby building on Freidrichstraße to serve as the official checkpoint, complete with a radio system. Now that it had a radio, it needed a call sign, and thus “Charlie” was attached to it. Within a few weeks, the US Army moved a trailer to the center of the road to act as the new control point on the Allied side. Checkpoint Charlie was now designated the major crossing point for Allied personnel, foreigners, and diplomats in the heart of Berlin. The Russians simply called it the “Friedrichstraße Crossing Point,” and their East German cousins the Grenzübergangsstelle (“Border Crossing Point”) Friedrich/Zimmerstraße—which was geographically where the checkpoint was located.