It looks like it could be a test unit for early digital PBXs (Private Branch eXchanges, usually used to interface between multiple extensions in an office building and one or more external telephone lines)
These usually have inputs and output for both analog (the conversation currently taking place on the outside line, hold music etc) and digital (transfer between extension, put on hold) data. Whereas modern PABXs usually have an RS232 serial port and a PC acting as a datalogger and constantly testing the unit, older PBXs such as the late 70s Monarch line used a TTL compatible digital i/o port.
So this looks like it may be a tool for engineers sent to fix PBX systems, as there appears to be no record button and only outputs for data/voice. It would probably come with a fixed tape supplied by the telephone company designed to output a fixed series of pitches over the analog lines and a series of common commands over the digital lines, so that the engineer could just set it up and wait for an anomalous result from the system, or give it an all clear if it was a routine test.