bill collector calling me at work

They asked for my son by name, who was not here, then they asked for me by name.  When I identified myself and asked why they were calling they said it was a personal matter.  When my son called the number back they refused to tell him who they were and why they were calling.

Everyday I get a call from this number  When I answer no one says anything and when I don't no one leaves a message.  I would like to know who is wasting their time and why are they continually calling.

Daily multiple calls.  Please stop the madness.