Clear-Com RS-603 and FL-7 flasher

I can't seem to make an FL-7 flasher work with the 2 channel RS-603 beltpack. The notes say it will work on Channel B, which it will if a single-channel beltpack is connected to Channel B and the call is pressed on that single-channel pack, but pressing the call button on the RS-603 will only illuminate the call light on the pack, it never activates the FL-7 flasher.

Anyone ever run into this and figured out a way to make this work?