davidvarley It maybe they are restricted from doing this by there parent company 02.
why would giffgaff be restricted?
giffgaff is a separate legal entity so it has its own business plans etc
the likely reason is that giffgaff is not a specialist international calling business, like lebara. However, you can call internationally numbers from your giffgaff phone but its not as chap as others like Lebara.