The busy indicator was a piece of cake to install; plug into the back of the Plantronics headset charger and you are good to go. However, I rated it only four stars because I thought it would light up automatically when I was on the phone as opposed to pressing the headset button.
https://www.amazon.com/Plantronics-Indicator-Voyager-compatible-65116-02/dp/B0002DODYO