Plantronics 855 is Bluetooth (Stereo) headset
The key feature of the Voyager 855 is that it turns into a stereo headset, as an additional earpiece attaches to it. This will allow you to listen to stereo music, coming from a phone or other device, featuring Stereo Bluetooth (A2DP profile). This will lead to additional 5 grams in weight (due to the cable) and total of 15 grams, which is 67% more than the 665 but still a good result.
source: Plantronics via Gizmodo