I thought the same. Where I live there's a register where you can suscribe, and if telemarketers call you then the company can be fined. Isn't there in USA such a thing?
Even if somebody calls using VoIP, they will mention their products, so I don't know why that would be an obstacle to identifying them.
"Telemarketer" nowadays means just anybody making annoying calls. I don't remember when was last time somebody who actually wanted to sell something has called. All the calls I am getting now are either asking for donations (which are exempt from do-not-call law, being a "charity"), doing a "survey" (also exempt) or are from criminals helping with my "credit card debt" , from "Windows security", from "IRS" etc. I imagine the criminals, already breaking more serious laws, would ensure they are not vulnerable to the do-not-call.
Even if somebody calls using VoIP, they will mention their products, so I don't know why that would be an obstacle to identifying them.