021 is not a country code. Numbers inside a country start like this: Agjabadi (Azerbaijan), Shanghai (China), Novi Sad (Serbia), Lausanne (Switzerland) and others. The first digit 0 is the trunk prefix for domestic calls and is dropped from abroad.
Have the full number? Check it in the number lookup.
| Area or operator | From abroad | Inside the country |
|---|---|---|
 Agjabadi · Azerbaijan | +994 21 … | 021 … |
| Beylagan · Azerbaijan | +994 21 … | 021 … |
| Hajigabul · Azerbaijan | +994 21 … | 021 … |
| 6 more in Azerbaijan | ||
 Shanghai · China | +86 21 … | 021 … |
 Novi Sad · Serbia | +381 21 … | 021 … |
 Lausanne · Switzerland | +41 21 … | 021 … |
 Belford Roxo - RJ · Brazil | +55 21 … | 021 … |
| Cachoeiras de Macacu - RJ · Brazil | +55 21 … | 021 … |
| Claro · Brazil | +55 21 … | 021 … |
| 28 more in Brazil | ||
 Parwan · Afghanistan | +93 21 … | 021 … |
 Algiers · Algeria | +213 21 … | 021 … |
 Split-Dalmatia · Croatia | +385 21 … | 021 … |
 Guantánamo Province · Cuba | +53 21 … | 021 … |
 Greater Jakarta · Indonesia | +62 21 … | 021 … |
 Tehran Province · Iran | +98 21 … | 021 … |
 Coachford · Ireland | +353 21 … | 021 … |
| Cork · Ireland | +353 21 … | 021 … |
| Cork/Kinsale/Coachford · Ireland | +353 21 … | 021 … |
| 1 more in Ireland | ||
 Beirut · Lebanon | +961 21 … | 021 … |
 Tripoli · Libya | +218 21 … | 021 … |
 Ulaanbaatar · Mongolia | +976 21 … | 021 … |
 Vodafone · New Zealand | +64 21 … | 021 … |
 Karachi · Pakistan | +92 21 … | 021 … |
 Fernando De La Mora, Lambare, Limpio, Luque, Mariano Roque Alonso, San Antonio, Valle Pucu and Villa Elisa · Paraguay | +595 21 … | 021 … |
 Bucharest and Ilfov County · Romania | +40 21 … | 021 … |
 Cape Town/Gordons Bay/Somerset West/Stellenbosch · South Africa | +27 21 … | 021 … |
| Vodacom · South Africa | +27 21 … | 021 … |
 Västerås · Sweden | +46 21 … | 021 … |
 Aleppo · Syria | +963 21 … | 021 … |
If a number starts with +, the first 1–3 digits are the country code. If it starts with 0 (8 in Russia and Kazakhstan), it is a domestic number: an area or operator code follows, and from abroad the 0 or 8 is replaced with + and the country code. The same digits can mean different places, so where the call came from matters.