can you explain how you dial IDD from the terminal?
here is an example from my set-up:
example, to call Mitel UK +44 1291 436 067,
9 is our exchange access
user dials 9 00 44 1291 436 067
if IDD is to be blocked, the users external digit barring has digit 00 barred. They can still dial other local or national numbers but not International.