I have Sony Ericsson W600i/S710 and HS850 works great. What we love  is folding boom, which saves lot of cell phone battery (fold it and turns off) When you get call just open boom and you are talking on headset. Voice quality is great in car (windows down) no background noise. It is best compare to Motorola HS801, HS820 and Tekkeon headsets we used in past. We just love how it is easy to turn headset off to save battery life of both devices. If you have problem with voice or connectivity you check your cell phone conversation volume (just press volume key when you are talking with someone to increase conversation volume just another way..)