All of Northern Ireland has the same area code: 028.
If dialling from outside the UK, use country code +44. For example, 028 9018 0123 becomes +44 28 9018 0123.
BT19 6TX
0035321 + the local number
The three-letter country code for passports issued by the Republic of Ireland is IRL.
To ring a landline in Northern Ireland from the Republic of Ireland, replace the area code 028 with the special shortcut code 048. More generally, to ring any number in the UK, including a Northern Irish mobile, from the Republic of Ireland, replace the leading 0 of the UK number with 00 44.
003531
Country code +35 is assigned to Ireland.
00353
Donegal is in the Republic of Ireland, not Northern Ireland. Its area codes are 071 and 074.
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028.
Telephone country code +44 is the United Kingdom, which includes England, Scotland, Wales, and Northern Ireland. However, there is no specific country code +4478. The country code +44 is followed by various area codes and local numbers to complete a phone number in the UK.
Northern Ireland is in the UK, and the international dialling code for the UK is +44.