[Windows 10 Mobile] nate0:
This is how I enabled AT&T US HD Voice/VoLTE on the Lumia 950 XL Dual sim and the factory Unlocked Alcatel IDOL 4s. It should work on the Lumia 950 Dual sim too, but I have not tested this method on it yet.

*Requires full file system access with MTP or SFTP using Interop tools

*All the files attached were taken from the latest AT&T Lumia 950 FFU (01078.00053.16236.35035 - OS 10.0.10586.318)

*To reset everything back to original factory settings requires either a re-flash or you can take the files you backed up before hand and replace them back.

**The one caveat to this is you need to use an AT&T Lumia 950 imei to register to your ICCID (sim card ID) with AT&T customer support to enable the hd voice/ims registration to come online. It might work with a Lumia 830 imei for AT&T too.

***There was mention of AT&T possibly whitelisting your device. This would allow IMS provisioning to complete without the need to have AT&T update your IMEI with that of a AT&T branded one. Seems even though you can ask AT&T to do this, it has not actually been proven to work. The only for sure way is to use a AT& branded Lumia 950 IMEI.

Update 7/20/2017: Updated files attached to include the specific ATT-US files from the 950 XL rom RM-1085_15919 (product: 059X7S5). The modem file is slightly different as it provisions with the same bands that come default already on the 950XL, it will not remove band 17 like the 950 ATT modemconfiguration.cab file does. The modem files as far as I know only work on the x50 Lumia models and not OEM models.

Install Interop Tools

- Here is the link. I used version InteropTools_1.9.437.0_arm_RC3.

Install Swish

- This sftp client found here (needed for Lumia 950/XL) for explorer sftp support. For the IDOL 4s enable Full file system access via MTP with read write access in Interop Tools under the Unlock options/settings.

Gain access to the file systems using Interop Tools app.

- In Interop Tools go the SSH--Account Manager and add an account. The account user name should be System.

- Tap the account System so it is highlighted and then tap templates to use.

- Tap enable full file system access with sftp. Then go to the template options and hit the authentication drop down setting it to password.

- Add the password (4 digit pin of your choice).

- Once your phone is on wifi, add an SFTP Connection under Swish. Set the user to system. Set the path to /c/. Once added double click it and login with the pin you set. Explorer will open with your phones files. There is a guide in more detail on sftp file system access made by snickler if you need to see more details about this method.

For the Lumia 950/XL do the below:

- Copy and back up all files under C:\Programs\CommonFiles\ADC\Microsoft and C:\Programs\CommonFiles\ADC\OEM.

- Then Copy and replace the files under C:\Programs\CommonFiles\ADC\Microsoft and C:\Programs\CommonFiles\ADC\OEM with the files of the zip file titled ADC.zip

- Copy and backup the file under C:\Programs\CommonFiles\OEM\Public\MMO\Multivariant.

- Then Copy and replace the file under C:\Programs\CommonFiles\OEM\Public\MMO\Multivariant with the contents of the zip file titled modemconfiguration.zip

- From there do a hard reset.

For the Alcatel IDOL 4s do the below using full file system access with MTP:

- Copy and backup all the files under C:\Programs\CommonFiles\ADC\Microsoft and C:\Programs\CommonFiles\ADC\OEM.

- Then Copy and replace the files under C:\Programs\CommonFiles\ADC\Microsoft and C:\Programs\CommonFiles\ADC\OEM with the contents of the zip file titled ADC.zip

- From there do a hard reset.

When the OOBE setup returns after hard reset, you may have a light blue/cyan theme and the display/dpi/font size may be a bit different too. This is due to the packages used from the AT&T 950. Go through the normal setup and let it fully boot to the start screen. Wait for the prompt to tell you to reboot your phone for settings/sim card settings to change/update.

Alternatively you can add the two files in the zip file titled MO_VoLTE_1117.zip to the c:\windows\system32. This will replace the MO_VoLTE_Label_1117.dll and MO_VoLTE_Description_1117.dll details found near the volte toggle under Sim settings with the proper descriptions.

To reset everything back to original factory settings requires either a re-flash or you can take the files you backed up prior and replace them back.

Thanks to the creator of interop tools [MENTION=7651894]gus33000[/MENTION]. Without access to that tool/app this and other things like this would not be possible. To [MENTION=2854207]snickler[/MENTION] for finding and explaining the sftp file system access in a guide to help others like me use that method.