To try and enable VoLTE turn off your phone, remove the SIM card and remove the battery (if possible) and wait for 20 minutes. Then reinstall the battery and SIM, and turn the phone on. To check if the phone has VoLTE, make a call and look in the top bar to see if the phone switches from 4G LTE coverage to 4G or 3G. If the phone switches out of 4G LTE, it is not using VoLTE
However, my understanding is the Treasure doesn't support VoLTE. I think you need to contact TracFone for confirmation.