I had the same experience as @Jeff in MD. Gave $10 to a door-to-door solicitor last year. Her issue was clean-up of Potomac. The daily calls continue. Thank heavens for Caller ID!
I had a call from this number after 9:00 pm Pacific time. Since I don't know anyone who would use it I didn't answer and the caller did not leave a message.
Haven't lived in DC for over two years. Didn't answer; they didn't leave vm. But, after reading these posts, I can trace it to a door-to-door solicitation to which I gave $10 about 4 years ago. Sheesh.