Hi everyone!

Background:

I have a Samsung Galaxy S21 Ultra Snapdragon Variant from Taiwan SM-G9980 128/12gb version. TGY/TGY,TGY/BRI. On One UI 4.1 March Patch. Vodafone NZ as network provider.

Problem:

Whenever the network mode is 4G/3G/2G and connected to 4G, the phone won't keep the VoLTE icon on which means that when I make a phone call connected to 4G, it would switch to 3G. When I restart the phone on this network mode, it would connect to VoLTE but after some time, it won't turn on again. I would check the sim status when the VoLTE is not connected and it would say that "IMS Registration: Not Registered" 

However, when the network mode is set to 5G/4G/3G/2G and the phone is connected to 4G, the VoLTE would just work normally.

Tried Solutions:

1. Contacted Samsung with feedback: feature llimitations because it's purchased overseas - not plausable as when I put a different sim card i.e. Spark, 2degrees, Skinny, it works fine.

2. Network reset, Factory reset, all the reset you can think of.

3. Contacted Vodafone to be told that it's Samsung's fault - still not plausable because works fine with other providers.

4. Sim Swap from store.

Regretting the purchase of the phone from overseas because of this but I'm hoping someone might have some troubleshooting I could try.