The native feature for every single device, user or other client is not yet available.
And I can't make any announcement until today if this is planned or not.
But what I can introduce is the Share Line Appearance, yes here Busy-On-Busy is available. Making S4B more look like an old fashion PBX :)
What it the Share Line Appearance.
This is a new feature introduced with the actual firmware from Polycom VVX and last Skype for Business Update November 2015.
Shared Line Appearance (SLA) enables you to configure a group of multiple devices that can each answer calls to a shared phone number (extension).
(SLA it is not supported for Skype for Business clients on computers, mobile phones, or other devices):
What does this mean to us. Well we can use busy on busy only within the VVX configured groups. But not yet between devices and client, or between soft clients.
The supported device are: