When a Latvian pensioner picked up a call from what appeared to be the State Police, the voice on the other end knew her bank, her account, and exactly what to say to make her afraid — because the people on the other end of the line had done this hundreds of times before.

Latvian and Ukrainian law enforcement agencies, on Wednesday, jointly announced the dismantling of an organized criminal network that used vishing — voice-based phishing, where callers impersonate trusted authorities over the phone — to defraud citizens across the European Union.

The joint operation, exposed the full machinery of a modern social engineering fraud which included call center operators in Ukraine, money mu...