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I bought a S9+ about 11 months ago and been on a Red Entertainment sim only plan. Recently VoLTE became available in my area but my phone couldn't access it due to being supplied by Carphone Warehouse and XEU rom. So now that Vodafone have released Android P I downloaded the latest Vodafone rom from Sammobile and flashed it. All seemed to go ok! Does the Galaxy S9+ show any signs that you are using 4G calling (VoLTE) other than when you make a call the phone connection indicator no longer drops down to 3G and remains connected to 4G? This is what happened on the few calls I've made since flashing and presume all is working as it should now my phone is using the Vodafone ROM?
How voLTE is displayed can be pretty much dependant on the firmware on the phone.
Some phones will display the voLTE icon in the phones status bar and some will as yours is just stay on 4G.
I understand what process you have carried out but I would also suggest to download the phoneinfo app from the Playstore just to make sure you've not tripped Knox thus voiding the manufacturing warranty.
Current Phone > Samsung Note 9 _512Gb > Model: SM-N960 Hybrid-Duos.
Samsung One Ui / Pie.
Samsung Gear s3 Frontier Watch.
Already have Phoneinfo app installed and haven't tripped knox. Flashed the Vod firmware using Odin and believe you generally won't trip knox doing so. Rooting on the other hand is a different thing. Flashing the Vod firmware not for the faint of heart! I opted to use 'home_csc' during the flash so that my data would not be wiped. There was a message when the phone first rebooted that said something about Vod UK not being fully active until the phone was reset? Something like that as when I 'clicked' it, it disappeared. Anyway Phone Info says the active CSC is VD2 - Vodafone Germany! That said it appears that VoLTE is working fine as is everything else. Two new apps have appeared, one being the Vodafone app 'Start' and the other being 'Mein Vodafone' which is the german version of My Vodafone. I already have the UK 'My Vodafone' installed and it's working as it should. I'll at some point reset my phone and see what happens. After 11 months or so a freshen up of the OS and a muck out of apps might not be a bad thing.
In our house we get a good 4G signal but 3 and 2G are very variable. I was missing some calls and some outgoing calls were failing due to this. I bought a SureSignal 3 years ago and this worked fine for quite some time though it concerned me that it always felt very hot. The other month noticed it wasn't working and upon checking it found it had burnt out. Some of the internals and the plastic case had melted though thankfully hadn't burnt the house down! So I've had VoLTE for 3 days and not missed a call or been unable to make a call. Each time the phone appears to stay on 4G where in the past it dropped to 3G. Ocasionally making a call both indoors and outside it would take ages for the phone to drop down to 3G and then make the call.
It's a bit of a going on that unless you buy your Samsung (and other Android phones) direct from Vodafone you do not get full access to their network services. I'll warn that flashing firmwares is not without risk and personally would rather not have to mess about doing such things.