Thanks @Cooler : I just opened another thread here, dedicated to installing /e/OS on my XZ3: Installing /e/OS on Xperia XZ3: VoLTE unavailable on stock ROM
@Taurus : the SIM card is inserted, and I restarted the phone multiple times, but the “Enable VoLTE” option still doesn’t appear.
I also tried with another carrier SIM card just in case, same result.
I’m wondering if it is risky or not to install /e/OS if this option has not been enabled.
I discuss this in the above mentioned link.
Thanks again guys,