You can't unless their info is online, but you really don't need it for yourself. However, you can and should report the phone number that is calling you. To both your carrier/service provider and the police. You can also block the number.
Again, we would have no way at all to tell you who it is that is calling. You can try searching online if you have the number that they are calling from. If not, then your only options are to either not answer if it is them or an unknown caller, block the phone number (if it shows), contact your carrier/service provider and report them, contact the police and report it, or change the phone number.