When I answer - there is silence. When I call back - it says its "Not in Service"!
I've been getting a few of these recently (some from the "Australian Contact Centre"). This explanation seems most likely:
I would bet it is a [contact-database-builder] checking to see who / what is on the other side of the line. By spoofing the originating information, they make tracing back to them difficult - and because it may be illegal for them to perform this practice in some jurisdictions, they make it harder to figure out who did it. They are mining numbers - you are the ore.