SD 835 doesn't support Dual 4G Volte. You can browse this thing on google.
SD chipsets that support Dual 4G Volte are only 636, 660 and 845.
@ Pikachu: that is because 835 is a old processor and 636, 660 are new processors. At the time 835 was launched there was no need of dual 4G volte so this thing coming in newer processors. Latest flagship 845 also supports this and some of the newer mediateks too.