I am using Xiaomi Mi Max now and my wife is using Redmi Note 3.
Whenever a call comes in, both phone's data connection will drop from 4G to 3G or even H+. This make surfing the phone while we are on the phone very slow. This problem is similar to the problem indicated at :
I called up Starhub yesterday but the person said that nothing can be done at Starhub side. But I chanced up on the case stated in the link above, so I believe there is some settings issue that Starhub has to do to allow both Xiaomi Mi Max & Redmi Note 3 to be able to run VOLTE.