I have been getting calls from this number 2-4 times a day for several months.  I never answer since my caller ID does not give a name.  Who are they?  What do they want?

I've been getting calls from this number often, sometimes twice per day.  I don't recognize the number and they never leave a message on my voice mail.
Today (Sunday), after receiving a call, I dialed back and the phone rang, I heard four blips and then a busy signal.  I'm thinking that this is a phone number that is not meant to receive calls, just to send them automatically.