When I started my career as a telecoms data engineer back in thr '80s the we used QBF to test modems on dial up data connections…the tester had a QBF button that would sent the text to a tester at the other end or it could even test round a loop back to itself.
You could inject errors with a button and these errors would count up on a basic 8 segment red led display to prove everything was working okay…happy days