Who called from 9310000000

Left no message

this number calls two or 3 times a day leaves no message!

Called 3 times so far today.. They hang up when you answer.. About to really make me angry

I have received calls for the last two days ALL day long from two numbers that hang up when you answer the first is 877-906-6829 and the other is 931-000-0000.  Does anyone have ANY info on these two numbers and why they are doing this.  It is REALLY beginning to freakin PISS me off.