I make certain to get the org.’s name right. Then I tell ‘em to send me a letter, First Class Mail, and I will send them money (assuming it’s a worthy org.) Then I tell them in no uncertain terms that if I EVER get another phone call from them they will have seen my last dime. I then hang up — hard!
I’ve used the Seinfeld method: Tell them I’m busy right now, but if they give me their home phone number, I’ll call them back. They sputter and stammer a bit, then say, “I can’t give you my home number”. I’ll say, “Why not? You called ME at home - why is THAT OK, but I can’t call YOU at home??”