Didn't answer it, but it was unusual.

just got a call from this number. it was a hang up. tried calling it back and got a busy signal

Me too SS

They are called Hilltop Research and they are looking for people to test products such as soaps, lotions etc.....They pay you cash apparently. They do have an internal "Do Not Call" list but you have to call them and request to be put on on it just like anyone company doing research.

I don't think you have to call them.  I believe you can make that demand during the call, and they have to put you on the list.  Check that out and see if they say you have to call them, ask them "what law requires that I have to call you?''