Unlike a traditional phone call, VOIP slices each call into millions of tiny digital packets, each of which can take a discrete route over the Internet. Surveillance equipment must either be installed permanently on a network or calls must be routed through FBI surveillance equipment before being delivered to the caller. “A call that is not [able to be intercepted] is a major public-safety and security dilemma,” says Szwajkowski. “Our tactical people are trying to plug every hole. But it’s like playing the field short one player””]
Source: https://taosecurity.blogspot.com/2003/04/bruce-schneiers-15-apr-03-cryptogram.html