A valid phone number in international format is as the following example
[country code] [network prefix] [number]
44 831 1234567
National dialing codes when calling internationally
In several countries, you may need to add a zero (0) when dialing a number inside the same country.
To dial a number in the United Kingdom, dial the internal area code (01296). The number would look like this: 01296 999999.
To call the same number from outside of the UK, it is not necessary to add the zero (0) before the area code. In this example, the number would be just 1296 999999. also you need to add country code and the number will look like 44 1296 999999. If you want to dial the same number from France, for instance, you would need to dial double zero (00) first, as this is necessary for France to initiate an international phone call. Putting it all together, in this case, makes the phone number 00 44 1296 999999.
How to dial an international number
In United States, you need to dial 011 before the country code while placing an international call however in Europe, you dial double zero (00) to place an international call . following are possible options to place an international call .
011 44 9999 999999
00 44 9999 999999
+44 9999 999999
List of International Calling Codes
93 --- Afghanistan
355 --- Albania
213 --- Algeria
684 --- American Samoa
376 --- Andorra
244 --- Angola
809 --- Anguilla
268 --- Antigua
54 --- Argentina
374 --- Armenia
297 --- Aruba
247 --- Ascension Island
61 --- Australia
672 --- Australian External Territories
43 --- Austria
994 --- Azerbaijan
242 --- Bahamas
246 --- Barbados
973 --- Bahrain
880 --- Bangladesh
375 --- Belarus
32 --- Belgium
501 --- Belize
229 --- Benin
809 --- Bermuda
975 --- Bhutan
284 --- British Virgin Islands
591 --- Bolivia
387 --- Bosnia and Hercegovina
267 --- Botswana
55 --- Brazil
284 --- British V.I.
673 --- Brunei Darussalm
359 --- Bulgaria
226 --- Burkina Faso
257 --- Burundi
855 --- Cambodia
237 --- Cameroon
1 --- Canada
238 --- CapeVerde Islands
1 --- Caribbean Nations
345 --- Cayman Islands
238 --- Cape Verdi
236 --- Central African Republic
235 --- Chad
56 --- Chile
86 --- China (People's Republic)
886 --- China-Taiwan
57 --- Colombia
269 --- Comoros and Mayotte
242 --- Congo
682 --- Cook Islands
506 --- Costa Rica
385 --- Croatia
53 --- Cuba
357 --- Cyprus
420 --- Czech Republic
45 --- Denmark
246 --- Diego Garcia
767 --- Dominca
809 --- Dominican Republic
253 --- Djibouti
593 --- Ecuador
20 --- Egypt
503 --- El Salvador
240 --- Equatorial Guinea
291 --- Eritrea
372 --- Estonia
251 --- Ethiopia
500 --- Falkland Islands
298 --- Faroe (Faeroe) Islands (Denmark)
679 --- Fiji
358 --- Finland
33 --- France
596 --- French Antilles
594 --- French Guiana
241 --- Gabon (Gabonese Republic)
220 --- Gambia
995 --- Georgia
49 --- Germany
233 --- Ghana
350 --- Gibraltar
30 --- Greece
299 --- Greenland
473 --- Grenada/Carricou
671 --- Guam
502 --- Guatemala
224 --- Guinea
245 --- Guinea-Bissau
592 --- Guyana
509 --- Haiti
504 --- Honduras
852 --- Hong Kong
36 --- Hungary
354 --- Iceland
91 --- India
62 --- Indonesia
98 --- Iran
964 --- Iraq
353 --- Ireland (Irish Republic; Eire)
972 --- Israel
39 --- Italy
225 --- Ivory Coast (La Cote d'Ivoire)
876 --- Jamaica
81 --- Japan
962 --- Jordan
7 --- Kazakhstan
254 --- Kenya
855 --- Khmer Republic (Cambodia/Kampuchea)
686 --- Kiribati Republic (Gilbert Islands)
82 --- Korea, Republic of (South Korea)
850 --- Korea, People's Republic of (North Korea)
965 --- Kuwait
996 --- Kyrgyz Republic
371 --- Latvia
856 --- Laos
961 --- Lebanon
266 --- Lesotho
231 --- Liberia
370 --- Lithuania
218 --- Libya
423 --- Liechtenstein
352 --- Luxembourg
853 --- Macao
389 --- Macedonia
261 --- Madagascar
265 --- Malawi
60 --- Malaysia
960 --- Maldives
223 --- Mali
356 --- Malta
692 --- Marshall Islands
596 --- Martinique (French Antilles)
222 --- Mauritania
230 --- Mauritius
269 --- Mayolte
52 --- Mexico
691 --- Micronesia (F.S. of Polynesia)
373 --- Moldova
33 --- Monaco
976 --- Mongolia
473 --- Montserrat
212 --- Morocco
258 --- Mozambique
95 --- Myanmar (former Burma)
264 --- Namibia (former South-West Africa)
674 --- Nauru
977 --- Nepal
31 --- Netherlands
599 --- Netherlands Antilles
869 --- Nevis
687 --- New Caledonia
64 --- New Zealand
505 --- Nicaragua
227 --- Niger
234 --- Nigeria
683 --- Niue
850 --- North Korea
1 670 --- North Mariana Islands (Saipan)
47 --- Norway
968 --- Oman
92 --- Pakistan
680 --- Palau
507 --- Panama
675 --- Papua New Guinea
595 --- Paraguay
51 --- Peru
63 --- Philippines
48 --- Poland
351 --- Portugal (includes Azores)
1 787 --- Puerto Rico
974 --- Qatar
262 --- Reunion (France)
40 --- Romania
7 --- Russia
250 --- Rwanda (Rwandese Republic)
670 --- Saipan
378 --- San Marino
239 --- Sao Tome and Principe
966 --- Saudi Arabia
221 --- Senegal
381 --- Serbia and Montenegro
248 --- Seychelles
232 --- Sierra Leone
65 --- Singapore
421 --- Slovakia
386 --- Slovenia
677 --- Solomon Islands
252 --- Somalia
27 --- South Africa
34 --- Spain
94 --- Sri Lanka
290 --- St. Helena
869 --- St. Kitts/Nevis
508 --- St. Pierre &(et) Miquelon (France)
249 --- Sudan
597 --- Suriname
268 --- Swaziland
46 --- Sweden
41 --- Switzerland
963 --- Syrian Arab Republic (Syria)
689 --- Tahiti (French Polynesia)
886 --- Taiwan
7 --- Tajikistan
255 --- Tanzania (includes Zanzibar)
66 --- Thailand
228 --- Togo (Togolese Republic)
690 --- Tokelau
676 --- Tonga
1 868 --- Trinidad and Tobago
216 --- Tunisia
90 --- Turkey
993 --- Turkmenistan
688 --- Tuvalu (Ellice Islands)
256 --- Uganda
380 --- Ukraine
971 --- United Arab Emirates
44 --- United Kingdom
598 --- Uruguay
1 --- USA
7 --- Uzbekistan
678 --- Vanuatu (New Hebrides)
39 --- Vatican City
58 --- Venezuela
84 --- Viet Nam
1 340 --- Virgin Islands
681 --- Wallis and Futuna
685 --- Western Samoa
381 --- Yemen (People's Democratic Republic of)
967 --- Yemen Arab Republic (North Yemen)
381 --- Yugoslavia (discontinued)
243 --- Zaire
260 --- Zambia
263 --- Zimbabwe
reference https://www.studyabroad.com/resources/international-calling-codes