I bought new BH-609 headset. I've tried it with E71, Lumia 900 and HTC One S and no matter with which phone the person I'm talking to complains about the low sound volume and not being able to hear what I'm saying. I've now tried everything that comes to my mind in order to do something about the problem. I've made those calls while being outside, in car and in quiet room but with no difference. With those same devices the older BH-606 works great with no issues with sound volume or quality. Perhaps I'm missing something with BH-609 settings...
Sorry to hear you have been having troubles with your headset. Have you tried changing the volume before connecting the headset to your devices, as this will be saved to the device once it connects.
If this does not change anything, please contact the retailer you purchased the headset from.
Let us know how you get on.