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
00353
Donegal is in the Republic of Ireland, not Northern Ireland. Its area codes are 071 and 074.
BT.. ....
028.
Northern Ireland is in the UK, and the international dialling code for the UK is +44.
Replace the trunk prefix 0 of the Irish domestic telephone number (mobile or landline) with telephone country code +353 (calling on a mobile) or 00 353 (calling on a landline). These instructions apply for all parts of the UK, even including local calls from Northern Ireland to places just over the border in the Republic of Ireland. Calls from anywhere in the UK to Northern Ireland just dial the UK domestic number. The area code for landlines in Northern Ireland is 028, with an 8-digit local number; mobiles from Northern Ireland use the same mobile prefixes as mobiles from the rest of the UK.
The telephone operator that uses the code 083 will most likely be located in the Republic of Ireland. The code 083 is used in Ireland and many other surrounding areas.