bᵻfˌoːɹ ðɪ ˌɪntɹədˈʌkʃən ʌv nˈæʃənəl dˈaɪəlɪŋ kˈoʊdz, ðɪ ˈɛɹiə nˈaʊ sˈɜːvd baɪ zˈiəɹoʊ twˈɛnti hæd ɐ lˈɑːɹdʒ mˌʌltɪpəl tˈɛlᵻfˌoʊn ɛkstʃˈeɪndʒ sˈɪstəm, kˈɔːld ðə lˈʌndən tˈɛlᵻfˌoʊn ˈɛɹiə.
Before the introduction of national dialling codes, the area now served by 020 had a large multiple telephone exchange system, called the London telephone area.