I’ve recently purchased a Gigaset GS6 directly to Gigaset and after installed /e/OS.
Previously tested some features and call phones showed indicator VOLTE on screen (Movistar).
After /e/ installation I have (with the same SIM) an indicator on the top of the screen “5G” and when I make a call phone, on the screen of phone app shows an indicator “HD”. I’m not sure if I making a VoLTE call.
“HD” means VoLTE in better quality than GSM (easy to achieve).
In general, VoLTE is almost mandatory today because there is only GSM fallback for voice calls after 3G shutdown.
From technical side, this is not so easy to implement because every provider “cooks his own soup” on this while voice calls were included in GSM and 3G standards.
You can get some additional information by dialing *#*#4636#*#*.
Then, the “Testing” menu appears, and you can choose “Phone information”.
Tap on the three buttons in the upper right corner. A menu appears, where you select “IMS service status”.
It then shows whether VoLTE or VoWiFi is available.
Additionally, you may change the network to 4G or 5G only and see whether you can make a phone call.
Finally, 2G phone calls and mobile data do not work simultaneously, which may help to get a first impression: If you have 4G/5G, your phone call starts instantly, and you still have mobile data (i.e., the 4G/5G indicator), then you are using VoLTE. If the start of the phone call takes 2 or more seconds and the mobile data indicator vanishes, you are using 2G (because the smartphone has to switch back to 2G).
EDIT: I have added a better description what to do in the “Testing” menu.