Anyone in any country can register a US Google Voice number
Thats not entirely true . The google number must first be registered to a verified US number . Than you can use it anywhere in the world . The number was probably registered by a US accomplice in Miami . Or possibly from a hacked google account .
https://support.google.com/voice/answer/115061?hl=enWhat you'll need to set up Google Voice
•Google Account
•Touch-tone phone, with a U.S. number •You'll select a type for each phone you add to Google Voice: Mobile, Work, or Home. (Text messages will only be sent to phones listed as Mobile.)
•Computer, with Internet access •Web browser: Google Chrome, Internet Explorer 8 and up, Firefox, Safari, Opera
•Flash 8 and up: Visit Adobe's Flash Player page to confirm you've downloaded Flash and what your version is.