为什么美国电影里的所有电话都是“555”开头的？

一直疑惑这是否是美国法律规定的，为什么有这样的规定。

In North America, telephone numbers with the prefix 555 are widely used for fictitious telephone numbers in television shows, films, computer games, and other media.

Not all numbers that begin with 555 are fictional—for example, 555-1212 is one of the standard numbers for directory assistance throughout the United States and Canada. In fact, only 555-0100 through 555-0199 are now specifically reserved for fictional use, with the other numbers having been released for actual assignment. How exactly this will intersect with the many uses of 555-2368 (one of the more commonly used fake numbers)[1] by AT&T and other telephone companies, remains unknown.

Area code 555 in the North American Numbering Plan is reserved for Directory Assistance applications.