got one of these calls myself and i have no idea who it is

I recieved a call from this number on March 21 around 5pm.  No message left. I don't answer calls I don't recongize and I don't return calls if they don't leave a message.  Unfortunately I recieve up 10-15 of those kind of calls a day but never a message.  Ringer is off, no bother.  My family and friends call me on my cell.