Well if SIM slot 2 either doesn’t have VoLTE enabled or is incapable (see @remote 's note on this), that could explain why SIM2 appears to work better than SIM1?
Well if SIM slot 2 either doesn’t have VoLTE enabled or is incapable (see @remote 's note on this), that could explain why SIM2 appears to work better than SIM1?