12-05-2017 09:53 AM - edited 12-05-2017 09:59 AM
You need cash credit on your account to pay for international texts or international calls - including UK/NI.
See here for the charges to the UK and other international destinations:
However if you dial an NI landline using the 048 prefix rather than the +44 ... UK prefix then it will be covered by your memberhsip or add-on landline minutes allowance.
Hope this helps.