@Tommy The 4G data network is not going away anytime soon. Even Verizon is allowing any device that is a Pixel 2 or later on their network. I just gave away a P2XL that I was using as a data device on Verizon with Sip for calls. So, you should be able to continue using them after the final T-mobile 2g cut-off (which had been postponed).
There is ongoing work to get VoLTE going, and Sip is available now with the experimental linphone app and the google voice web app. You should join the NA Issues UT group on Telegram. We can help you better there.