Hi John,
Apologies for the delay in replying.
The setActiveLink API is specifically for the Savi 7xx multi-line device. Using with other products such as the D100 will cause an exception/error.
For a single interface device you can programatically make the link active by setting AudioState to MonoON, and turn it off by setting it to MonoOFF.
If you have "audio sensing" enabled on the D100 in our Plantronics Hub software, then the line is activated automatically based on the presence of audio on that playback device. In this case your calls to set AudioState to MonoON/MonoOFF will be ignored (but it does not hurt to have them there, to cover the case where audio sensing is disabled.
Hope it makes sense.
Thanks,
Lewis.