There is no add-on for this.
The only way to do this is to put cash credit on your account to pay for international calls.
The relevant charges are listed here:
You also need an active membership plan to make such calls.
Note that Northern Ireland landline numbers called using the 048 prefix are covered by your membership plan call minutes allowances:
If you need to contact 48 support about this then see the links in my signature below.
