FYI now the law allows telemarketers to call cell phone numbers. You can add your cell number to the do not call registry but it will take anywhere from a few months to a couple years for it to be published in the next do not call registry that these telemarketers use. (in other words they will ust claim your number isn't in their do not call list until they actually get sued)
Caller ID is easily faked, I'd warn people not to press any keys when anything is calling you you never know but you might get a phone bill for hundreds if not thousands of dollars from offshore numbers which are outside the reach of US laws.