The insecurity comes from the fact that once a call is in the network, it is mostly passed off without validation or verification. You only need to find someone

The insecurity comes from the fact that once a call is in the network, it is mostly passed off without validation or verification. You only need to find someone willing to carry your call in to the network, and the rest takes care of itself. To change the routing of a call other than yours would require you to access a carrier's systems and change where the call is routed to--which is substantially more difficult.