I have a Samsung Galaxy S21 Ultra Unlocked phone which I bought from US. I enrolled to One UI 4 beta in US. The software had been trouble free for me until I had to move back to India.
My carrier in India is Airtel which has good LTE coverage in my hometown. It supports VoLTE and WiFi calling as well. Since I moved to India, VoLTE or WiFi calling is not working for me. I rolled back to One UI 3.1 using Smart Switch and VoLTE and WiFi calling started working again. But, since I was already enrolled into One UI 4 beta, I got upgraded to the beta version again and VoLTE and WiFi calling stopped working. Interestingly, I used my Mint mobile sim card which I used in US and VoLTE and WiFi calling work with this card.
I contacted @USBetaModerator and he deregistered me from One UI 4 beta. I was able to downgrade to OneUI 3.1 again using Smart Switch. I was so relieved seeing VoLTE and WiFi calling working again.
Today, One UI 4 stable version was rolled out. I upgraded to this G998U1UEU4BUK7 version. To my frustration, VoLTE and WiFi calling stopped working again. This means that anyone who has a Samsung S21 series phone (or maybe any Samsung phone) with OneUI 4 will have the same problem. You won't be able to use it outside US.
I am assuming the network auto switch feature which Samsung introduced with One UI 4 is causing this issue. I had been trying to get some help from the Samsung team, but none has been able to help so far.
@sr14 is another user who is facing the exact same issue.
Attaching the screenshots for everyone's reference. As you can see, in One UI 4, "IMS Registration" status is shown as "Not Registered" and "Voice over LTE" toggle is "OFF". In One UI 3.1, "IMS Registration" status is "Registered" and "Voice over LTE" toggle is "ON".
Users outside US: Please don't install OneUI 4 until this issue is fixed.
I reset network settings at least 3 times while I was in OneUI 4 beta and it didn't help.
Anyways, I reset network settings again and VoLTE and WiFi calling still don't work.
And for everyone's information, I factory reset my phone while in One UI 4 beta. Started everything fresh. Still VoLTE and WiFi were not working.
I get LTE. But, voice calls get connected to 2G network. Mobile data and calls won't work together.
To some relief, I am not using a Jio sim. Jio has only 4G coverage and doesn't have 2G support. @sr14 has Jio sim and his phone is more like a paperweight to him.
Definitely. But, it's been 2+ months since the first beta was out. I expected better from Samsung. They shouldn't have released the final version before the issue was fixed.
I am pretty sure that many S21 series owners who bought phone from US who are currently outside US will be facing the same issue now (now that OneUI 4 is officially released) and wondering what's going on.