958 is used in NYC and possibly other verizon areas...
but as the other poster said 1-800-444-4444 is the one most commonly used, as 958 does not seem to work on all landline numbers any more.... I also have used 1-866-MY-ANI-IS
the toll free ANAC numbers are better than the 958 (or other code used by your local CO) because they can be used not only for landlines, but also cellular and VoIP (although depending on what your VoIP provider sends as ANI may not be accurate)