Today I got another call from 617-588-0005 today.
I seem to be getting a lot of them, but I finally tracked down who and why. You see I don't ordinarily pick up calls from people I don't know due to telemarketers. But whoever claimed to be calling from 6175880005 was persistant, and so I answered.
It turned out to be a friend who lives in Europe. Yep, it is a US phone number, but he was calling through Skype. It looks like Skype is using 6175880005 for calls from those that otherwise don't have a real phone number.