My Ooma is set for some numbers to go to voicemail if they are on a certain list. I got a notification of a message from Ooma and the phone  did not ring so I know they were on the list. I get calls like this all the time. I think the calls are routed to telemarketers and they wait until someone answers. Somehow this records them waiting, I have heard all kinds of crazy things in the background while they are "waiting" for me to answer. Not sure if it is any organization I support or normally just asking for a donation. Uness they leave a message they are SOL.