Whether you're dealing with actual law enforcement or someone pretending, the procedure is the same: Say nothing, tell your lawyer and wait patiently to be arrested. Assuming it's real, it's the action of a court, and court proceedings are routine, slow, procedural, and consist of paperwork. And somebody has to show up physically with a warrant. For chrissakes, make them work a little for it. If they can cow you without even getting up from their chair, you're making it too easy. This just happens to deflect scammers too.