VoLTE (Voice over LTE) delivers voice calls over the internet. It is a digital packet voice service with many benefits for mobile operators and consumers.
Here are three main benefits for consumers:
– supports higher quality calls
– connects calls much faster (lower latency)
– improved battery life.

You can use VoLTE only if your phone and network support it.
My mobile network (Telekom Slovenija) supports it and VoLTE enabling is easy in the web dashboard:

The second part was enabling the VoLTE feature on the phone. Almost all modern phones support it, but some of them don’t have this feature in the menu. My OnePlus 5 didn’t show it (this depends on some reasons), but I knew that it is supported. Here is a simple tutorial on how to enable it.
The process is quite simple but be aware that something can still go wrong 🙂

UPDATE (4th August 2021): Bellow described procedure don’t work on the latest version of OxygenOS 10.0.1

1. Open the dialer on your phone and type *#800#
You have to use the original dialer, not 3rd party like Google Dialer!
This will start a special OnePlus app which is usually used when OnePlus service team try to help you with problems. Don’t worry, press Enter to start the magic 🙂
2. Click “oneplus Logkit”:

3. Click “Function Switch”:

And then restart your phone. This is important!
After the phone comes to life, go to “Settings -> Wi-Fi & Internet -> SIM & Network” and there should be a new feature (or two of them if you enable VoWifi beside VoLTE):