That one just started to call me to, but I reported them as a harassing caller to my local phone company. Under FCC rules, unknown/anonymous callers are "harrasment" - hit: *57 and wait for the prompts from your phone company to initiate call trace. They'll tell you if the trace was successful. The telephone lines belong to the phone company, you lease them, but they are legally obligated to investigate harassment on their lines when reported. It'll cost a $1, but that's well worth it. The phone company is then in the position of being able to follow through with law enforcement. What they cannot do is tell you WHO called, but its worth it to cost them money and aggravation of prosection. :)
People have been shocked before when Police have shown up at their doors with warrants over telephone harrassment and there are also Federally mandated fines. Ignore it and be too cheap to spend a $1 (it goes on your monthly phone bill) and it won't be initiated - you do have to follow through with the complaint by calling their Harrassment Investigation line, too. But worth it to get these obnoxious nitwits to put up or shut up, don't you think? Its in the front of your local phone book or on your local phone company's website. No secret. Even the FCC site has a complaint process and they actually follow through. I've complained about AT&T not processing payments for 5 days and got copies of all the correspondence. :) Scare the idiots corporate, crook, etc. The law applies to them too.