This is essentially a portable Adds 580 terminal. It does have a builtin accoustic coupler, which probably works with 110 and 300 baud 103 dialup modem protocols.
Since this has been around the LA area for probably 40 years, it is interesting that the filter material on the back isn't powdered. I will remove and replace it.
Photos from ebay sale ended 5/30/2016
As nice a specimen of this terminal as I've seen.