It's the National Wildlife Foundation. I am a member, donated and they have every legal right to call and ask for more. If you don't want them to call you again, simply ask to be removed from their calling list or block the number if you have that option on your phone plan. NWF are a great group of people with limited funding so try cutting them a little slack.
Well, National Wildlife Fund won't be getting any more donations from me. If they want to call me, why can't they leave a message instead of my having to go to all the bother of calling back the number to see who it was.