021 is:

• in Brazil, the telephone area code for the city of Rio de Janeiro and surrounding cities (Greater Rio de Janeiro)
• in China, the telephone area code for the city of Shanghai.
• in Indonesia, the area code for the city of Jakarta and surrounding cities.
• in Iran, the telephone area code for the capital city of Tehran.
• in Ireland, the area code for the city of Cork and a surrounding part of County Cork.
• in Libya, the telephone area code for the capital city of Tripoli.
• in Nepal, the Nepal Telecom telephone area code for the district of Morang.
• in New Zealand, the "area code" for cellular phones on the Vodafone network.
• in Pakistan, the telephone area code for the city of Karachi.
• in Serbia, the area code for the city of Novi Sad.
• see Telephone numbers in South Africa.
• in Sweden, the area code for the city of Västerås; see Telephone numbers in Sweden.
• in the United Kingdom, the former area code for Birmingham England (changed to 0121 on 16 April 1995).