Excerpt from: 2600, Spring 1990, Page 27:

In some places, scanning has been made illegal. It would be hard, though, for someone to file a complaint against you for scanning since the whole purpose is to call every number once and only once. It’s not likely to be thought of as harassment by anyone who gets a single phone call from a scanning computer. Some central offices have been known to react strangely when people start scanning. Sometimes you’re unable to get a dialtone for hours after you start scanning. But there is no uniform policy. The best thing to do is to first find out if you’ve got some crazy law saying you can’t do it. If, as is likely, there is no such law, the only way to find out what happens is to give it a try.

It should be noted that a law making war dialing illegal was passed in Colorado Springs, CO. It is now illegal to place a call in Colorado Springs without the intent to communicate.