I called the number back and got a recording that this was Gateway Communications. It said that they help lots of charities make contact with their supporters and that if I had received a call from their number it was because they were trying to contact me on behalf of a charity that I supported or had supported in the past. I do support numerous charities, so this is likely legit. However, since I did not answer the call, I don't know the specific charity they were calling on behalf of.