While this will vary from country to country, at least here in the US there are several situations where the legal means are not more effective. The main one is scammers who just don't care, either because they're running a "hit and run" type of operation or because they're operating from overseas. Then there are classes of phone calls which are (again, at least in the US) exempt from the legal "do not call" rules: politicians making campaign calls, and survey firms, which the politicians use to determine who to bother calling. It's a nice loophole that they've constructed for themselves. Luckily it's only a problem for a few month every other year.