Based in Singapore, we are not able to make phone calls if we set to LTE only via *#*#4636#*#*. How do we activate voLTE on our phone? Brought from Asus @ Bugis. But when using 2G / 3G / 4G as network preferred type no issue with phone calls. But mobile data connectivity is not stable at 4G / 4G+ keep getting H / H+. Help please, 1 Week old only the phone. No root.