Do I have to have data mode on for making VoLTE calls?

Nope, it’s not necessary to have data mode on, however you do have to have VoLTE option enabled from your handset.

Use VoLTE!

To be able to experience VoLTE, both the caller and the receiver must have the following: 4G SIM that’s been inserted in first SIM slot or the SIM slot with data capability and network mode s

Which iPhone models will support VoLTE?

iPhone models above 7 will all support VoLTE.