The best response, is no response.  Just ignore them.
Many of those who work as Debt Collectors are not polite because they are not Boy Scouts and/or Girl Guides.  They are mainly a downtrodden lot, and their ranks include ex-convicts, drug addicts, 20-somethings without high-school diplomas and a variety of other hard-luck cases.
Oh, my Gawd, they are like thugs.  However, the more clean-cut types simply can’t get the job done.  So don't expect that debt collector, who is calling you, to be nice!!!