They are probably looking for the person who had that number before your kid got it. Or they may be looking for some low life who lied about his phone number when he took out a car loan. Either way, keep the phone in your pocket for a few days. Answer the phone and talk to them. Explain that the person they are looking for isn't part of your household.
Thats how I got rid of them. Of course that was after assigning them a "no ring" ring tone that sent them to voice mail for 2 weeks. Over that 2 weeks they called up to 6 times/day.
It was wells fargo, unless you are a current customer and have debts you should not be getting called by this number, PEOPLE GIVE OUT RANDOM NUMBERS TO CREDIT COMPANIES (YOUR NUMBER) AS A RESULT YOU GET CALLED BY DEBT COLLECTORS, call the number it back, you may have to wait a while on hold, but eventually someone will answer, tell them the situation and hopefully they will be as helpful as they were for me.