“NPA” stands for “Numbering Plan Area.” I can’t even find any references in google to “national prefix area” except this thread and places where the three words occur accidentally (like “national prefix” at the end of one sentence and “Area” at the beginning of the next).
I think you are conflating the 555 NPA (“area code”) here with the 555 NXX (prefix or the first three digits of the seven number code).
The 555 NPA is reserved for “directory assistance services.” It is not currently in use. (Click here and type 555 into the search box.) Long distance directory assistance can be reached by dialing 1-NPA-555-1212, not 1-555-XXX-XXXX. For example, directory assistance in New York can be reached at 1-212-555-1212.
The 555 prefix within each geographic NPA (not including the Service Access Codes such as 800, 900, etc) is reserved for regional and national information services, except for numbers of the form NPA-555-01xx which are reserved for fictitious phone numbers.
Incidentally, here is a list of all 555 NXX numbers that have currently been assigned. Remember, 555 NXX numbers are intended to be used across multiple area codes. Although the numbers in the above list have been assigned, it may be difficult to reach them since most of them are accessible only when using a specific carrier.