VoLTE will work over 5G (and sometimes over 3G in esoteric configurations). It is very data centric, even using SIP for call control. A rule of thumb is, any time you have a data connection, VoLTE can be used.

This is why Wi-Fi Calling and VoLTE took off at roughly the same time. Once you can route calls as data, you can offer it over both cellular and Wi-Fi. (Wi-Fi Calling has an additional outer IPsec layer, but it is the same "guts" inside the tunnel.)