I'm doing some research on VoIP Security - Thought you would like this one for the good old fashioned PSTN...

Most people think that the standard weapon used by most private investigators is a gun. The fact is, the most powerful weapon a private investigator uses is the telephone. Knowing all you can about them when practicing your profession is important. It's the telephone, not a gun that is the most important piece of equipment a private investigator uses. The telephone brings you both business both new and old and is the one major communications medium you obtain information with. Product knowledge on it's use as it relates to your profession is of extreme importance. This article gives you some general but little known tips about private investigators use of their major weapon, the telephone. Some of them employ little techniques that are quite useful well others require special equipment. In the case of those that require special equipment, I'll lead you right to the web page for review.

Follow this link for more...http://www.pimall.com/nais/n.20telcotips.html