They are passed as a flag, and also in the text field of the callerid data. (Not all callerid devices will display the text field, as BT abandoned using it for peoples' names very early on after a trial in one area.)
There are a number of occasions when BT proves additional info in the text field of the callerid data, as I described earlier. (Actually, I missed one out which I've seen, RINGBACK, which also mirrors the ringback call type in the callerid data.)
That's how the BELL system works. BT sends the data before the first ring, preceded by a line reversal. There's also an off-hook signalling method which is used during call waiting, but that requires that the phone indicates it can take the data when off-hook, or it's not sent.