Hi, I have never really though of those brand devices as all that good. I recommend Konftel, they are simply the best. With Aethra, a close second...last is .......
Just get the top of the line one, and you will NOT be disappointed.
http://www.telecom.co.nz/content/0,8748,101437-201060,00.html
BTY, I expect that your unit is faulty, the electret mic's aren't very good, and fail quite a bit, as with all cheap electret's, and also check positioning too, this can make a difference too, make sure nothing is around to reflect the sounds as this is what makes it echo and sound bad, regardless.