Calls and then hangs up. If you answer they know they've got a live number and will call back under a different number soliciting services. Unfortunately no way to really block the number. They are calling from non VoIP or web based calling so they can change the incoming caller ID number at will. Best not to answer if you can and then block. Probably have to do that multiple times. Usually calls back with 1 number changed. They most likely give up on calling you after a while from no answer rather than giving up due to being blocked.