Each button is from a module whose name contains the provided letter exactly once. Taking the index of those letters yields a number — a two-digit number in cases where two buttons are shown — which translate into letters:
|USA Maze, Regular Crazy Talk||U||14||N|
|Digital Cipher, Error Codes||D||18||R|
|Ice Cream, Bartending||I||18||R|
Reading both the provided and the obtained letters together, the clue phrase THREE HUNDRED THIRTY NINE appears. Combined with the flavour text, which hints at the Maritime Flags module, the solution is the callsign whose associated bearing is 339.