It could be something as simple as confirming that you have an active number. Be on the look out for an increase in calls from operations displaying other numbers.
I would also advice you not to answer unfamiliar callers. Nor should you return their calls.
Jon, it is more than likely someone verifying live phone numbers. They then put the live numbers on a list which is then sold to the really bad guys to start calling. They are all sewer rats.