What cisco products would I need to set up a voip intercom, where from a buzzer signal interpreted as an incoming call, live video of the caller would display on the cisco phone device.
You would need to locate a standards-based (H.323 or SIP) video intercom device. Cisco does not manufacture them. From the phone side of things you can use any video-capeable endpoint. A 7985, 9971, 9951, CUVA, etc are all options.
Here's one example of a video intercom from CyberData:
