I think they use Internet and IP spoofing to mimic a phone call.
At a national level of resources, such calls can be traced back to their origin in a matter of seconds if the right monitoring equipment is already in place. Frequent targets of the swatters may have such arrangements setup by friendly authorities. You just need the right friends.
But wait! There is more. All such calls are logged and can be traced retroactively for days if not weeks afterwards. It is a little harder in that situation to get the exact point of origin, but swatters believe they are untraceable, so they repeat their activities.
Repetition, and bragging about what they are doing gets them eventually, no matter how clever they are.
If the Government really wants to find swatters, they can find them.