The most important thing is to add the international dialling code of the country.
Dial +352 to call a Luxembourg number, +33 to call a French number or +32 to call a Belgian number.
The emergency number 112 is free.
VoLTE activation on an Android smartphone
The Voice Over LTE service is automatically activated if you have an offer and a compatible Android mobile (provided you have updated your phone to the latest version).
VoLTE activation on an iPhone
If you have an IOS mobile, you will have to follow the activation process.