Crazy debt collectors keep harrassing me for someone else.?

Someone who had my cell phone number before me owes money to “American Reading Club” he doesn't have the # anymore. Thus, for the past four months I’ve been receiving harassing phone call from Pre Legal Office on behalf of American Reading Club. These debt collectors call my phone more than three times a day and ask to speak with Derrick A., whom I don’t even know. Every time they call, I tell them that I don’t know that person and they have to stop calling my phone but they will not stop. Indeed, they tell me, ‘’If you really want us to stop calling, you have to make Derrick A. pay the amount of $ 625.58 balance he owes for the magazines he ordered in 2006 from the American Reading club.” Clearly that is impossible because I have no relationship with this Derrick .At this point; I really don’t know what to do put an end to these calls. Many suggest that I change my phone number, but I have had the same number for 2 years. Is there anything else I can do in order to end these harassing phone calls?

Please help me !