I'd leave this number alone entirely, even with an answering machine if possible. Not likely this guy has any good intent, despite claiming to be taking donations for police. I've been amazed at all the absurd donations I've dealt with through the years until I quit taking unknown calls. Good chance this clown's fishing for financial info and to see if your number is live for resale. If you must deal with it, tell them you gave at the office, ask him if he's the police, and ridiculous questions. I'd just ignore it all the way around.
Overall, probably an ill-advised move even taking the call, much less telling them to remove you from their system. While you might never hear from this number again, don't be surprised if you're not slammed with more idiotic calls. They might not get what they're hoping to get, knowing your number is a working number is sweet success for scammers and spammers. IGNORE THE PHONE.