Try 1234, if not, then you will have to ask BT to send it in the post. Usually you would have been informed of it when you ordered the service.
From BT Website
"If you forget your call barring PIN you will need to contact BT for a reset. In some of our exchanges the Call Barring service locks after 3 failed attempts. If you suspect that this is the case then you will need to contact us on 0800 800 151"