[Honor 4x ROMs, Kernels, Recoveries, & Other Develo] pndwal:
Well, I like your enthusiasm too, so no problem :) . However:

VoLTE has generally only been implemented by providers on phones they supply and include the settings for it in their branded ROMs. The VoLTE spec does provide for ways to set up other phones for VoLTE, but carriers have generally not used this protocol, instead using their own custom setups. Hence they generally only support a limited number of phone models they supply. This has been the case at least up on N but may have changed with O as this OS makes VoLTE implementation easier, however carriers are stow to change their methods.

This means working VoLTE is rare on custom ROMs, even those for devices with SoCs supporting VoLTE such as our Snapdragon 410. For example, we don't have it for Ascend Y550 despite Vodafone promising to implement it on stock ROM for this device.Turns out we apparently need Android 5 for VoLTE, and seems Vodafone never updated above 4.4 despite their PR reports of 2015 saying Y550 would support VoLTE. If they had, I'm sure someone would have made it work on LOS. And this is the case with most devices with custom ROMs. They usually never had VoLTE set up on earlier stock ROM, and this in not the fault OEMs like Huawei, rather the carrier didn't implement, or the phone was bought unbranded. When these do have working VoLTE it generally won't work if users switch to another carrier either, due to the custom nature of VoLTE setup.

If you want to try to make it work, read the following-

Regarding VoLTE setup on Custom ROMs:

You will most likely need Magisk module VoEnabler (Experimental - adds settings to build.prop to enable both VoLTE and VoWifi), allows users to try to enable VoLTE, but no-one reported success for some time even with these critical changes made.

When I tried it on Y550, the result was that when I check phone Tester (dialer, *#*#4636#*#*, tap Phone Information, scroll down) I see

VoLTE provisioned, Video Calling provisioned & Wi-Fi Calling provisioned all toggled on

. (Were all toggled off before installed Magisk module VoEnabler.) However, when checking IMS Service Status (3 dot menu, top right), I still see:

IMS Registration: Not Registered

Voice over LTE: Unavailable

Voice over Wi-Fi: Unavailable

Video Calling: Unavailable

UT Interface: Unavailable

So seems IMS registration is still a hurdle to enabling VoLTE. Also, may need IMS settings in APN (or an IMS APN), and these changes will require information from carrier.

The good news:

More recently experimental VoEnabler Magisk module has actually allowed users to add support for VoLTE/VoWiFi on a number of previously unsupported devices!!!

Read the Magisk notes for this module under Magisk 'downloads' regarding 'possibly making it work'. Apparently enabling VoLTE/VoWiFi is easier with O, and you could play with the Module and try making changes mentioned in the other threads that allowed it to work. Might be a bit complex, and I haven't had time to play with it again, so I mention it so others can try. Let us know if you do.

Hope this helps and that you give it a shot. At least you know it's provisioned at your carriers end. Please report on any trials :) . Regards, PW.