I have been using a f(x)tec pro1 for years on 4G+VoLTE on their network, and got the same text message saying they would terminate my account if I did not get a different phone. I decided to lay low and moved my sim into a supported phone with the intention of letting the dust settle white I prepare to port my number to t-mobile (they have by far the worse service in my area, but seem to be the only US provider allowing the pro1 to be activated).
I still have an inkling of hope that I will be able to move my sim back to the pro1 after whoever is responsible for the 3G shutdown gets their bonus and they stop going after unsupported devices aggressively.