This number has been showing up on my caller ID since about the middle of April. Same scenario - they call, they don't leave a message. They are calling about once a week, but it appears to be picking up a little bit.
I have my phone settings to send this caller directly to voicemail, no ringing my phone!
To the person on the other end of this number - please leave a message with a valid phone number to reach you at. I'd probably call you back.