You have bumped a thread that has not seen a response since September. During the 10-year anniversary part and livestream on [Sailfish Community News, 4th November](October 14), we were given confirmation from Joona Petrell that we would be getting VoLTE and a new device in 2022 (Xperia 10 iii).
I will say that at this time, on Mint Mobile, I have not experienced degraded calls due to a lack of VoLTE, so you night consider a different MVNO, or be patient and wait for the next software update.