Retronetworking

I'm quite familiar with FrameRelay as used over PRI interfaces. In such setups, there is only FrameRelay, with one fat HDLC pipe over 30 concatenated timeslots. There is no D-channel, and there is only FR on the entire PRI.

However, I recently read in a German textbook that there was Frame Relay also for BRI lines, and that it was provided in parallel to the normal ISDN/Q.931 signaling. Basically there would be both LAPB and LAPF? Does anyone know more about this? Was it ever deployed anywhere?