I have this phone since 2 weeks, purchased on gearbest. It is simply UNUSABLE for calls. I have the same problem that many other users have noticed: the microphone is inverted. When I receive calls, the person who is calling me can not hear my voice when I talk on the mic on the bottom of the phone. I can be heared only if I speak on the mic on the top camera, ore use some headphones with mic.
Since I paid for this phone, I expect that a firmware that fix the problem will be released VERY soon. Otherwise I'll spread the existence of this problem as much as I can, with bad reviews on purchising sites and youtube videos.