It turns out it's a (prototype) Caller ID simulator. Not a receiver but a transmitter. Plug a phone or CNID display into it, and you can send a ring signal and a Caller ID packet to the device to test it out. I'm sure they built it so people designing receiver devices could verify they worked. But it's just as useful for testing, say, software that takes action based on incoming caller ID, like my long-deferred fax/voicemail server upstairs.
I didn't really get this call: