The telephone system, in the US at least, could charge people extra for calling a number with the extra charge going to the person/company that was called (those famous 900 numbers, now gone, that kids would call and rack up $10000 phone bills.) It seems to me that it would be pretty easy to set up a system where, if a person is not in my contact list, they get charged some amount of money to call me. Setting it at 5 or 10 cents would be enough to kill almost all of these spam calls and not deter a real person from calling you.