Radio host Rush Limbaugh said his apology to the Georgetown law student he called a “slut” was sincere but also joked that he, too, got a busy signal Monday when he called the show to join the growing roster of advertisers abandoning it...AOL and Tax Resolution Services...became the eighth and ninth advertisers to leave Limbaugh’s three-hour show and at least one radio station dropped the program as he sought to stem the exodus of advertisers and fellow conservatives declined to offer him support.