From 1922, the first three digits of a seven-digit subscriber numberin other words, the local exchange codeswere represented with letters by way of a mnemonic.
fɹʌm nˈaɪntiːnhˈʌndɹɪd twˈɛnti tˈuː, ðə fˈɜːst θɹˈiː dˈɪdʒɪts əvə sˈɛvəndˈɪdʒɪt səbskɹˈaɪbɚ nˈʌmbɚɹˌɪn ˈʌðɚ wˈɜːdz, ðə lˈoʊkəl ɛkstʃˈeɪndʒ kˈoʊdswɚ ɹˌɛpɹᵻzˈɛntᵻd wɪð lˈɛɾɚz baɪ wˈeɪ əvə nɛmˈɑːnɪk.