You don't. The carrier software to enable it is only installed on their carrier-locked (contract) phones that you purchase through the carrier.

 

My own non-carrier Pixel 2 and my daughter's non-carrier Galaxy S8 both support WiFi calling out of the box, and my daughter uses it with EE (proving that there's no special EE software needed). I've also tested it, but as I have no coverage problems I have no need for it.

So I'm afraid that the previous answer is incorrect: it is not only available on phones where the carrier has added it, and it works perfectly well with non-carrier handsets on EE. Other networks may do things differently, and so @jMave's reply may be a correct description of how things work with their carrier, but this is first-hand experience with the same network you are using. However I cannot say whether a non-carrier LG Velvet includes it, as I've never seen that handset in real life. If the phone software doesn't support it you are almost certainly out of luck.

I might add that the last time I checked EE only supported WiFi calling with pay monthly SIM-only contracts and not with pay-as-you-go SIMs (all of my family's phones are on pay monthly SIM-only contracts and we all use non-carrier handsets).

 

Yeah it's the old blame game.
Good luck with any resolution

 

The easiest solution is to just use a Wi-Fi calling app.

Personally, I use Dingtone.
Why?
Because it was the easiest one to use and it is totally free.

You do need a good Wi-Fi connection to use it on Wi-Fi, but it can also work over the data connection to your cell provider.

It gives you a voice over internet protocol (VoIP) phone number that actually is a real phone number.

Esentially, this gives you another line for your device, and it an also be used to use a tablet as a phone.

Up to 5 devices can share a single number.

The hitch?
Credits.
Credits can be purchased, and are fairly cheap.
Credits can also be obtained for free by watching short ads.

Checking in each day?
3 free credits.
Ads vary between ½ and 1 credit, depending on the ad.

And there are multiple other ways to earn free credits as well.

I have used Dingtone for more than three years, and it has not cost me a cent.
I got to pick my number from a long list of available numbers.
(Custom numbers are available.)

I started out with no cellular service, only using a smartphone to access the internet over Wi-Fi.

I found Dingtone, and it made it possible for me to use the device more like a normal cellphone.

Now that I actually have cellular service, I keep Dingone as an extra line, and it is excellent for that.

https://dingtone.en.uptodown.com/android

 

EE will only know that WiFi calling is enabled for your account. If you bought the phone offline they won't know about its software. So I'd say that the issue is plainly with LG: does the phone support it, and if so why can't you find the setting?

For the record, on Pixel it's in Settings > Network & Internet > Mobile Network > Advanced (under "Calling"). But settings vary between manufacturers and versions.