You will find that there are many times when the human voice fails to get a message through to another person. It could be noise, especially in HF radio, or dialect, or maybe you just had your teeth repaired and the freezing has your tongue snoozing. 

I often find that I have trouble being understood on a phone. I'm in good company, many people are plagued similarly. During war years people on boats and aircraft were working under stress and awful noise from the machinery they were within. Over a period of time a number of ways to express individual letters and numbers were developed with many changes over time. In amateur radio we tend to use a method adopted by the United Nations - the ITU

A   Alfa         B   Bravo         C  Charlie

D   Delta        E   Echo          F   Foxtrot 

G   Golf         H   Hotel         I    India      

J   Juliet       K   Kilo          L   Lima

Y   Yankee       Z   Zulu