My Zoiper on Iphone couldn't send my voice out. But my colleague's are fine, they call each other and both way sound works fine. Only when I call anybody else, the other side can not hear my voice, while I hear the other side well. The statistics shows no incoming data rate at all on other side Iphone.
As the tutorial suggests, I checked the 'Privacy' settings and try to disable and enable 'STUN', nothing helps.