በቀዝቃዛው ጦርነት ወቅት አንዳንድ ዝቅተኛ-ተደጋጋሚ የሬዲዮ ጣቢያዎች በተከታታይ ድምፆች ወይም በሞርስ ኮድ ውስጥ ጥቂት ተከታታይ ቁጥሮችን ያሰራጫሉ ፣ ይህም በውጭ ሀገራት ለሚሰሙ የስለላ መኮንኖች ነው ተብሎ ይታመን ነበር።
During the Cold War, some low-frequency radio stations would broadcast a few series of numbers in synthesized voices or Morse code, which were believed to be for intelligence officers listening in foreign countries.