I have about a dozen Adtran ISU 128 ISDN 'modems'.
They are surplus from Bell South and work.

Why on rec radio swap?

Well the are dandy for remote rig control for equipment that uses RS232.
Range to 18,000 feet over cheap telco twisted pair.
Manual is on the Adtran Web Site.
Briefly, one is set up to provide the clocking which is normally
supplied by the telco for ISDN lines. Hook up your pair and you're ready
to go.

What about the audio?
Well you can use a pair of DSL lowpass filters, one at each end, and rub
your audio over the same pair! Can run DC as well if you want to operate
a relay to power up the remote end. All on two wires.

$50/pair plus ship from 30114.

-bob