Each kana is assigned a code word, so that critical combinations of kana (and numbers) can be pronounced and clearly understood by those who transmit and receive voice messages by radio or telephone, especially when the safety of navigation or persons is essential.
There are specific names for kana, numerals, and special characters (i.e. vowel extender, comma, quotation mark, and parentheses).
Thus, to convey ba (ば), one would say "hagaki no ha ni dakuten (はがきのハに濁点)".
To convey pa (ぱ), one would say "hagaki no ha ni handakuten (はがきのハに半濁点)".
Thus 1, 7, 4 are pronounced hito, nana, yon rather than ichi, shichi, shi which could easily be confused with each other.