If you know that the number you're dialing is a detonator, why isn't the phone call itself illegal? If it doesnt detonate, doesn't that make the call an illegal act itself (attempted murder) and if so, why isn't a call resulting in successful detonation illegal by the same reasoning? IANAL but I'd think the former would be just as illegal as the latter except that the successful phone call would be lumped into the greater charges of terrorism/homicide alongside any violations for actually building and placing the explosive device.

I don't know much (or anything) about the prevailing criminal justice philosophies among the judiciary but this sounds like a question that can only be answered by a thorough review of the criminal code and precedent. Wire fraud, for example, is one of those felonies that you can be guilty of in the process of committing other more serious crimes so obviously some components can be broken out of the larger crimes.