I pronounce it with the 'Keh' sound when I use the phonetic alphabet, and I've heard others do so as well. The word doesn't need to start with the sound of the letter, it just needs to be somewhat intelligible as that word, and not one of the other words in the phonetic alphabet. For example, if Celine was spelling her name, she would use the phonetic 'Charlie', even though her name doesn't start with the 'Ch' sound.