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Flashing AT&T Firmware on Unlocked S9+ G965U1

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FAUguy

Senior Member
Mar 1, 2011
921
137
South FL
After using an Unlocked S8 U1 on AT&T for the past few years, I decided to get the S9+ (since I don't care for the S10).
I order two S9+ (renewed) from Amazon: the AT&T S9+ model (G965U) and the Unlocked Model (G965U1), since I wanted to see if VoLTE and WiFi Calling would work on my Pre-Paid plan.
The AT&T G965U does work fine with VoLTE and WiFi Calling, but the unit has some nasty fall marks on it, and the volume buttons don't work well, so it's going back.
The Unlocked G965U1 only has a few minor screen scratches, but overall is in better condition than the AT&T G965U, but VoLTE and WiFi Calling doesn't work.

So I'm not sure what to do.
I can clearly hear the difference in Call Quality between the model with VoLTE and without (HSPA calls).
It's too bad AT&T still won't allow VoLTE on non-AT&T Branded Samsung phones.

I have read countless posts on S9/S9+ Firmware and CSC flashing with ODIN (which I'm familiar using) but I can't find a definite answer on if there's way to take an Unlocked S9+ U1 and flash it so that VoLTE will work. I know WiFi Calling won't work, since the IMEI has to be on AT&T Whitelist.

As of right now, the S9+ is on the latest firmware G965U1UEU7CSJ3.
 

FAUguy

Senior Member
Mar 1, 2011
921
137
South FL
Just to give an update. I was able to flash the S9 Plus G965U1 Unlocked model with the AT&T CSI5 firmware as shown in the Guides section. After that, the S9+ was able to use VoLTE and WiFI Calling on my AT&T Prepaid plan.
I had also ordered a (used/renewed) Note 9, and when I got it, it had Verizon firmware on it. So with it I had to flash the Note 9 first to the U1 Unlocked firmware and then to the AT&T U firmware.
So with both the S9+ and Note 9, I can get VoLTE and WiFi Calling to work.

Now I just have to decide which one I want to keep, as the S9+ was $348 and the Note 9 was $390.
I like the S9+ since it's not as heavy and more comfortable to hold, and it seems like the (smaller 3500mAh) battery lasts longer (not sure why). But I like the features of the Note 9 (S Pen) since I had the Note 2/3/4/5 years ago and miss those features.