Scammers use names of legitimate business; however, if they won't leave a message I wouldn't believe the caller is legitimate. You can always look up the number of the legitimate business to see if the numbers match. But if you are on the DO NOT CALL list, as I am, they shouldn't be calling you in any case.
With a little more research, Clothing Pickup appears to be a real charity and their website shows they have a clothing pickup box in our area. I am still blocking as we have other charities we use and I am ok with one call from a charity and a brief message, but not repeated calls with no message.