When calling Northern Ireland landlines, you can use the normal UK +44 prefix, or you can use the 048 prefix described here:
https://en.wikipedia.org/wiki/Telephone_numbers_in_the_Republic_of_Ireland#Calls_to_Northern_Ireland
A call to NI using +44 is charged as an international call at 50c per minute.
http://www.48months.ie/membership-plans/international-calls
Calls to NI using 048 are covered by your plan or add-on bundled landline calls allowance.
So if you're calling an NI landline, it makes sense to use 048 rather than +44.
Refer to the wikipedia link above for instructions on how to use the 048 prefix (basically you replace the 028 prefix in an NI landline with 048).
