ʌntˈɪl mˈeɪ nˈaɪntiːnhˈʌndɹɪd nˈaɪnti, ðə zˈiəɹoʊ wˈʌn kˈoʊd kˈʌvɚd ðə sˈeɪm ˈɛɹiə æz ðə kˈɜːɹənt zˈiəɹoʊ twˈɛnti kˈoʊd ænd hæd kəpˈæsᵻɾi fɔːɹ fjˈuːɚ ðɐn tˈɛn mˈɪliən tˈɛlᵻfˌoʊn nˈʌmbɚz.
Until May 1990, the 01 code covered the same area as the current 020 code and had capacity for fewer than 10 million telephone numbers.