It appears that the document you referenced is in error about the functionality. The spec sheet makes no reference to BLF functionality for the 1625; yet it does for the higher end model 1628 &1630. I also checked the FW release notes thinking that perhaps at one point the function may have been there as while I have a number of these installed, I have never noticed such…but then I use them as line keys as Lucas indicates. In any event, I found no reference to the function having ever having been there or reference to its subsequent removal.
I can only suggest submitting a ticket to support or returning (hopefully) and getting a model that meets the need.