Who called from 4129811972

Called my cell phone(a number I never give out) and when I answered, nothing, dead air. Who are these people and are they going to call again?

another robocall on my cell from 412-981-1972 caller ID said "NO NAME" on Friday 09-05-14 @ 1042 AM

missed call on my cell.....did not ring.
call went right to my BLOCKED call list.
unknown SCAM

another robocall on my cell from 412-981-1972 caller ID said "NO NAME" on Monday 09-08-14 @ 404 PM

missed call on my cell.....did not ring.
call went right to my BLOCKED call list.
unknown SCAM

Even if you answer there is no one there. Why are they allowed to call?

It was probably Penn Hills, not Ben Hills. I'm from that area and have never heard of Ben Hills.