Who is this number

This number calls and nobody is on the other end of the line.  Maybe they are using a web-service that allows you to enter any number in caller ID (I once had 000-000-0000 and another time 999-999-9999 and verified that this is how they did that)...

Then again, when I return the call it tells me that the mailbox if full so it sounds like a legit number of some kind, but what service would use 310-999-9999 and why wouldn't I be able to find it online?

This number has called several times and it remains silent for about one minute then hangs up.