The 08 numbering range was previously used for calls to Northern Ireland, but following the UK's renumbering of Northern Ireland in 2000, this changed, so to call a number in Belfast from the Republic:
Northern Ireland
Web Site
Northern Ireland - Ireland - Europe Phones
Northern Ireland - Ireland - Europe Phones
Northern Ireland Europe
Northern Ireland 2024 Proud Andy Waterworth hails Northern Ireland U17s after easing past Republic in Euro qualifier Belfast Telegraph
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Northern Ireland v Belarus: Nations League match to be played at Windsor Park BBC.com
Northern Ireland vs Belarus to be played in Belfast as planned after visas granted - The Athletic The Athletic