Refer to figure.
ICAO Annex 10 Volume II
5.2 RADIOTELEPHONY PROCEDURES

5.2.1.3 Word spelling in radiotelephony. When proper names, service abbreviations and words of which the spelling is doubtful are spelled out in radiotelephony, the alphabet in Figure 5-1 shall be used.

The Radiotelephony Spelling Alphabet is used primarily to ensure clear and precise communication over radio. Each letter of the English alphabet is assigned a specific word to ensure that each letter is clearly understood when spoken, particularly in noisy or poor communication conditions. This helps to avoid confusion that can arise from similar-sounding letters. The call sign HB-YKM corresponds to the following words:

H → Hotel
B → Bravo
Y → Yankee
K → Kilo
M → Mike