In the US, the law specifically exempts registered charities, political candidates, and "survey canvasers" from the prohibition against calling a number listed on the do not call registry.
If a candidate wants to call you it is perfectly legal.
If a solicitor for a charity wants to call you it is perfectly legal.
If a survey taker wants to call you it is perfectly legal.
A little on the shady side, and ambiguous with respect to whether it's really legal, is the call from a bill collector who just "thinks someone who owes money" is at the number.
About the only ones barred from calling you are businesses who might want to tell you about what they'll do for you if you have not done business with them recently. If you respond to any call, even an illegal one, in almost any way (such as by saying $@%#^!!) the business may presume that a "business relationship" has been established, which permits them to call you at will for the next 90 days. Since they are permitted to call for up to 90 days from the date of your last "business relationship," a call on the 89th day that gets any response technically entitles them to another 90 days.