I have found a solution from Zangi
I have tested their app, and found following advantages:
Audio calls normally work even with bad internet 2G/EDGE.
I was surprised with video calls quality. On good internet connection it's a real HD, with 720HD resolution. But it works on bad networks too.
On Google Play Market the app has high rating(4.5) and good reviews.
It works only on mobile devices: iOS and Android.
There are no group chats and conferencing.
The product is a new one and google finds few information about them.
Does anybody know them? Has anybody worked with them?