Interesting find: IBM 5866 modems

I’ve come across a large number of old IBM 5866 multiplexing modems. From what I gather from playing with one, they allow several serial connections (from dumb terminals?) to pass over one phone line to another such modem, where each signal is demultiplexed and sent to an appropriate destination (the serial ports of a timesharing system?) Each port appears to allow a maximum thorughput of 14,400 bps. Now for the big question:

How do I attach one of them to the telephone network? There are no phone jacks or anything indicating what the ports on the back do. The following is what I think each port is:

4 25-pin serial ports (female) for dumb terminals. There are also two blank plates that probably cover additional serial ports, for a maximum of 6.
A 9-pin female serial port with a white plug in the center hole in the top row of pins, perhaps to attach to a telephone interface module.
A power socket
A blank panel that, when removed, reveals a large empty bay with a short white connector at the top of the opening, probably to add an expansion module.

I’ve played around with the panel and learned how to switch the modem from primary to secondary mode, as well as changing other settings. If I press any of the buttons on the bottom row that appear to control dialing, the display reads “no feature”. The reference card that slides from under the unit informs me that those buttons control SNBU, whatever that is. The card also summarizes error codes and status messages.

Searches on Google and IBM’s website haven’t turned up any useful information. Any help in identifying the unit’s precise function and its use is greatly appreciated.

Note: Please don’t tell me “you need newer equipment” or “don’t bother with that old stuff” or any variant thereof. Procuring and using old IT equipment is a hobby of mine.

Sample AON/SNA SNBU Modem Configurations

astro’s link may be sufficient, but here are several other links on the IBM product site using “5866” AND “modem”:
IBM search results