"911 service not prepared for new generation of pranksters":

After paying a small fee to one of the readily available caller ID spoofing services available on the Web, a prank caller with a grudge or a serious psychological problem can call 911 and tell the operator just about any story he or she wants. Since the 911 system wasn't built with VoIP in mind, these calls appear to originate from anywhere, and said hooligans take full advantage of the opportunity. The practice has been dubbed "swatting," typically because the spoofed emergency stories that these troubled individuals make up are horrible enough to send police and even SWAT teams to unsuspecting victims on the other side of town or the continent.

Here is one example cited by the article.