When the ancient Greeks adopted the alphabet, they had no use for a letter to represent the glottal stop—the consonant sound that the letter denoted in Phoenician and other Semitic languages, and that was the first phoneme of the Phoenician pronunciation of the letter—so they used their version of the sign to represent the vowel , and called it by the similar name of alpha.
wˌɛn ðɪ ˈeɪntʃənt ɡɹˈiːks ɐdˈɑːptᵻd ðɪ ˈælfəbˌɛt, ðeɪ hæd nˈoʊ jˈuːs fɚɹə lˈɛɾɚ tə ɹˌɛpɹᵻzˈɛnt ðə ɡlˈɑːɾəl stˈɑːp—ðə kˈɑːnsənənt sˈaʊnd ðætðə lˈɛɾɚ dᵻnˈoʊɾᵻd ɪn foʊnˈɪʃən ænd ˈʌðɚ sɛmˈɪɾɪk lˈæŋɡwɪdʒᵻz, ænd ðæt wʌzðə fˈɜːst fˈoʊniːm ʌvðə foʊnˈɪʃən pɹənˌʌnsɪˈeɪʃən ʌvðə lˈɛɾɚ—sˌoʊ ðeɪ jˈuːzd ðɛɹ vˈɜːʒən ʌvðə sˈaɪn tə ɹˌɛpɹᵻzˈɛnt ðə vˈaʊəl , ænd kˈɔːld ɪt baɪ ðə sˈɪmɪlɚ nˈeɪm ʌv ˈælfə.