This number usually appears as an Unknown caller because the phone lines that are associated with this number are from a high-security office. If you know someone that works for the government or Boeing or some other high-security company, it could be that they're calling you so pick up!
This is a phantom number used as a mask for security purposes for Boeing's (and perhaps other companies) phone lines that require they be secure. My mother works for Boeing and when she calls from work my home phone caller ID comes up with *Unknown* but my cell phone caller ID says *310-343-6099*, however her direct line at Boeing is not this number, in fact not even the same area code. I asked her about it and she said it was something to do with their phone line/number security. So if you get a call from this number and know someone who works at Boeing or any other high security facility, it is likely them who is calling you.....Hope that helps...
These scumballs are General Contractors doing work in the area using a spoofed phone number trying to rip off the consumer. Don't give them the time of day.