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:

Module name(s)LtrIxLtr
Alphabet T8 H
Binary Puzzle R5 E
Simon Speaks E8 H
USA Maze, Regular Crazy TalkU14N
Digital Cipher, Error Codes D18R
Algebra E4 D
Color Math T8 H
Ice Cream, Bartending I18R
Stack’em, Equations T25Y
Alphabet Numbers N9 I
Adventure Game N5 E

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.