If I put a VoLTE/LTE SIM on my 3G handset, will it work?

No, you have to have a 4G handset WITH VoLTE option. Ensure you and the other person on the call has the following to experience VoLTE:

- 4G SIM (skitto is already 4G, just ensure the other caller also has 4G SIM)
- VoLTE enabled handset with the option turned on
- updated OS software of your phone