Who called from 6616702588

This number keeps calling, hangs up if I answer the phone, and does not leave a message if I don't answer.

calls me all the time....often have static on my machine, but no voice.

Company called Donor Serivices that tries to collect donations for organizations such as Zoological Society of San Diego

Received call, when I answered, there was ~5 seconds of silence, then disconnect.

This number has been calling our land-line repeatedly for four days at random daylight and early evening hours.  No way has been found to ask the callers to take us off their list, even tho we are on the do-not-call list.  
Is there proof this is association with the San Diego Zoo?