I've never seen it, but could it be the phone is running plugged in? Just a guess maybe you have the phone connected to your Mac or plugged in...Easy to check.
Plantech voyager pro. I spend half my day on conference calls, its really comfortable for my little ears. Not the cheapest at £50 odd from amazon but worth the money for me personally.
Had a plantech before this which fizzled out after about 18 months. They replaced it no quibbles.