One trick the telemarketers use goes something like this:
Company "Phone" is a telemarketing service.
Company "Roofers", "Vacations", and "Realtor" sign with them to drum up business.
"Phone" calls you and asks if you need roof work. You say no and ask to be put on their do not call list. They tell you that they won't call you back about a roof.
The next day "Phone" calls to talk about vacations...

The next day "Phone" calls to talk about real estate... etc. etc.
Tell them you don't want ANY calls for ANY reason and ask what company they work for. You'll get a total runaround. It's all a joke to them, registry or not. My best advice is to speak real softly. When they turn their headsets up, try to take out their eardrums.

I always get a kick out of it when they comment on my language. My standard response is: "Then stop calling you stupid <insert "C" word>". I use it whether the caller is a guy or girl. Sometimes I
like when they call. It helps me blow off steam. If they don't treat this stuff seriously, why should I?