It seems like it's the same people who call regularly with a 425 number.  If you answer the phone, they ask if you want to know about jobs that are available, and then hang up before the end of the first sentence.  It seems that their goal is to get you to say anything (beyond your "Hello"), so they know it's a real phone number for a real person.  Then they keep doing the call on a weekly basis with another 425 number.  The second call and beyond are never from the same number.

So I'd like to know who they are and why they're doing what they're doing.  Perhaps they could just be selling your number to other bad people.