Today I've played a little with the native SIP. I found out that the file "android.software.sip.voip" is missing. I pushed this file to etc/permissions, changed permissions to rw-r-r and voila! - after rebooting the missing internet calling part appeard in my dialer settings menu. I managed to set my voip provider credentials and enabled SIP. I called a few numbers and apparentlly got the connection (the called person confirmed it and i can saw it in my voip billing list) but nobody could hear the other one and dialer was frozen. it is not the common SIP issue, i can use CSIPSIMPLE without any problems. I'm attaching the missing file if somebody is interested in further research regarding native voip on moto x.