1. You should be able to enable it in the settings. For me, I can't activate VoWiFi on my Samsung Galaxy s7 edge. There's no option for me to activate it.
2. Simple words, yes. Specifically, neither normal or not normal. 4G or 4G+ depends on the coverage and surrounding factors.
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"4G" : Connected to only one 4G LTE frequency band (e.g. 1800MHz).
"4G+" : Connected to 2 or more LTE frequency bands (e.g. 1800MHz+2600MHz), also known as CA (Carrier Aggregation). When you see 4G+, that means you're on LTE Advanced network instead of the conventional LTE network.
For example, it could mean that when you're indoors, your phone can't connects to both 1800MHz+2600MHz bands like when you're outdoors. In your situation, your indoor reception for 2600MHz may be too weak or can't be detected, so it connects to other bands that is still detectable and usable, yet as a result, only 1 band are readily available in your particular indoor area, which is 1800MHz, so your phone continues to connect to that band and appear it "4G" instead of "4G+". Or, perhaps only 900MHz band are available in your indoor area, so it'll appear as "4G" too.
Because don't forget, if the building doesn't have in-building signal repeater, that means you're receiving the signal from outside of the building. The lower the frequency, the better the signal penetration to building walls and objects. Which explains the characteristics of 2600MHz band has poor wall penetration, unless the cell tower or repeater is very nearby.
If in-building signal repeaters is available in the building (e.g. shopping mall), and the repeater transmits in both 1800+2600MHz bands as well, then you'll get 4G+ in that building.
Digi doesn't operate in 850MHz. 850MHz band is only used by Unifi Mobile for now, and it is one of the band they use.
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Frequencies used by all 4 big telcos are 900MHz (Band 8), 1800MHz (Band 3), 2100MHz (Band 1) and 2600MHz (Band 7).
For Digi's 2100MHz, it is mainly used for 3G everywhere, but 4G is also used in this band in certain indoor places (in-building signal repeaters) and underground (e.g. Underground LRT stations of Kelana Jaya Line).
1. I couldn't find VoWIFI/WIFI Calling in the settings though, do I need to ask the CS to enable it for me? Cuz I'm pretty sure most Huawei phones on Android 8.0 do support VoWIFI.
2. Previously it was fine on my RedOne, it showed 4G+ most of the times indoors (at home), but Digi only shows 4G, that's why I asked. And my mistake on the 850 mHz part, I was actually referring to 900 mHz, just that I got messed up by the numbers after I checked this article which was from years ago.
But thanks for the brief elaboration, learned quite a lot about these network stuffs.