CH_ASUSI believe that the last statement is not true. On my ZF6 I cannot make voice calls via an AT&T 4G cell. AT&T has not included the ZF6 on the list of compatible phones after 3G phaseout
Stay tuned for when Zenfone 8 officially arrives to the US.
We've certified VoLTE for both ZF6 and ROG series with AT&T in the past year.