If you do not choose a long distance carrier when you establish your ISDN phone line you will not be able to make a long distance call unless you use one of the 1010 (also known as a dial-around) number. Even if you do have a long distance company assigned to your line you may still have some reason to use a dial-around number.
It appears as though a digital ISDN long distance call is handled differently than a voice call. Because of this, some long distance carriers may not work when trying to place a long distance call using their dial-around network.
User "Roamer1" had this information in a forum post:
1010321 (Telecom*USA = MCI) works, but you can only do 56k data calls, as 1010321 calls are trunked differently than regular MCI customers who are PICed to (101)0222 (please do NOT dial this unless you are PICed to MCI as you will be GOUGED.) You can try 1010345 (Lucky Dog = AT&T) but I'm not sure if that will even work.
Most of the cheaper dial-arounds (1010811, 1016868, etc.) don't support ISDN data at all.
If you want to do DOV and not normal ISDN data, the only carrier I've had any luck with is AT&T.