To update this post...
Thank you, Polycom! This has been adressed in the recently-releasted UC firmware 5.9.0, as described >HERE<
I hope to see more comments as we start using the new firmware!
Just wondering if 5.9 or 6.0 allowed you to setup transfer to voicemail of a BLF contact (I saw your previous solution with adding secondary contacts on a 2nd page of a sidecar, but it's less elegant as would need some extra configuration).
Danny, just wondering how you got it working on 5.9.x
I had to use version 6.0 that shows "Lines" in the EFK prompt (no additional configuration other than transfer2voicemail softkey" that we need anyways).