I think my problem is due to the transition signal change from LTE to 3G when there's incoming call. Suppose to change smoothly, but when there's incoming call, I can see the signal change to 3G but the phone nve ring. Singtel tech team advise me to only use LTE when I need it. N switch to 3G so I can receive calls. They said they're looking at 4G voice.
Anyway, if u guys have any problems. I highly recommend u all to go to their singtel support twitter n communicate with the ppl there. They are more dedicated than the 1688 helpdesk. Was quite surprised with the effort n quick response n the knowledge plus the follow up they have there, instead of the 1688 help desk. Previously I called 1688, only ask me to reset network settings. Zzz