[Windows 10 Mobile] nate0:
I never fully tested the 830 since I soon gave it away after checking it. IMS registered on it and held even after a reboot. I never tested Wifi calling though. So For the 830 you can try this below, and this is how I would do it on any Nokia phone that I found was compatible.

The x50 series Microsoft phones use modem cab files and the older x20 series Nokia models use different ADC provisioning files (dll's and xml's) that I have not quite figured out yet...

Unlock full file system access. Take the zip file provided and unzip the contents somewhere on your PC. Included is a folder called ADC with a Microsoft and OEM directory and files inside. Also included is a folder under \OEM_nvi_version_updated\Public\ called NVI with two T-Mobile modem nvi files that I have updated the version number on to force an nvi update and two other nvi files that are the default ones for T-Mobile from the 640.

Navigate on your 830 to the directory c:\programs\commonfiles\ADC. Backup the files there just in case. Then copy paste the Microsoft and OEM directory from above into the ADC folder on your 830 replacing any files you are prompted to replace.

The next steps will update the NVI settings in the modem and will hold on a hard reset too, but you need to know which variant default is set for your region model. To find out your default variant defined by the NVI folder go to c:\windows\packages\RegistryFiles\ and with 7zip file manager open OEMSettings.reg in that windows right click the file OEMsettings and edit with notepad or another text editor then search for MultiVariant and the entries below in the example picture will help you know the default NVI variant files to replace. In the picture below for this case I would replace the files under c:\programs\commonfiles\OEM\Public\Nokia\MultiVariant\15133\NVI. Make sense

[IMG="Registry MultiVariant entries"]https://xdaforums.com/attachment.php?attachmentid=4435431&stc=1&d=1520008655[/IMG]

So navigate to c:\programs\commonfiles\OEM\Public\Nokia\MultiVariant\. In there should be some variant folders defined by a number (ex: 15818, 15133 or 12012 etc.) and there might be more than one. Copy all those and back them up some where just in case. Replace the NVI files under the folder Default and under your Variant Default number (ex: 15818, 15133 or 12012 etc.).

To update the nvi settings quickly, take the NVI folder I listed above inside the \OEM_nvi_version_updated\Public\ directory that you unzipped and paste that inside the path c:\programs\commonfiles\oem\public. So you should have more than several directories under c:\programs\commonfiles\oem\public including an NVI directory with the updated version nvi files inside.

[IMG="NVI folder"]https://xdaforums.com/attachment.php?attachmentid=4435432&stc=1&d=1520008655[/IMG]

Once all this is done run thor2.exe -mode uefiflash -ffufile <your 830 ffufile> -do_full_nvi_update -skip_flash -reboot

This will reboot the phone and once it is booted again will prompt you it needs to restart to apply settings. let it restart on its own and check if IMS registers now and you should have bands 1, 2, 4, 5, 7, 12, 17, 28 available I believe. Make sure 2, 4, 12 and the GSM frequencies are there.

Lastly, there is a file in the zipped attached folder called nwp_network_settings_db_targets.xml. You can put that under C:\Programs\CommonFiles\OEM\Public\Nokia\MultiVariant. It contains the MCC MNC info for carriers, the 830 should already have one and it might not make a difference on yours since you have the Unlocked model. I also have found this file is very similar if not exact on most models. But I included it anyway if you want to replace it since it is part of the T-Mobile 640 LTE model.