Call back gets nothing but a continuous ring. A legitimate company or office, including a legitimate debt collector would have a header identifying themselves. This is probably one of the 5 prominent scams running around these days. Here's a USA today article on these.
http://www.usatoday.com/story/tech/columnist/ ... scams/31213423/
Make it your policy to NEVER answer calls from unknown persons or numbers you don't recognize.