Simple solution... don’t pick up the phone (assuming you have Caller ID) if you do not recognize the caller. If they really want to talk to you they will leave a message.
The only problem with that is I have children in college and they sometimes will call using a school phone or friends phone. I’m always afraid to ignore a call in case it’s an emergency.
When I attempt to return call in the evening after work, using the number on my caller id... I receive a "cannot connect" type message... basically there's a call block not allowing the donation company's call center to receive calls (they can only make outgoing calls). Therefore I have to be home and actually answer the phone to tell them "take my number off the list."
I hate these companies... I wish a Mt. Etna-type volcano would open up under the call centers and melt them away.