Hello Bill,
I was unable to find a replacement that is VoLTE compatible. I was in chat with support asking if they offered a VoLTE replacement and from the limited information I could provide they stated the existing device would still be supported. If you can StraightTalk support you can provide your plan information and they will be better to advise you on what to do. Click contact us at the bottom of the page and use the live chat feature to ask them.
I hope that helps.