@carl So you might consider changing your device if you still wish to continue with UT beyond July 31 / 2025, as Pixel 3a has nearly 0 chance to handle VoLTE. Or move to another carrier to gain some time.
As @wally said above, there's no official VoLTE time table right now at UBports, but things are moving well and some crucial steps have been done those last weeks.
This is what I've been able to understand from the various community channels - and with all due caution - as I can be wrong:
Volla 22 and Volla X23 have already jumped in since OTA-8. Some users would probably need to upgrade their Halium port from 9 to 12. (No idea how to do this though...)
Fairphone 4 / Fairphone 5: Already running on FP5 (dev mode). Should land with Noble on the FP4. Both did provide pretty decent results. 5G/NR bands should also come at the same time. .
Redmi Note 9S / Pro / India: It happens right now. Our gorgeous team has just merged VoLTE into devel (both Focal and Noble) for further tests. Seems to run pretty well, including SMS over LTE and Bluetooth car audio calls. High chances to land within the next Stable release. (Big big Kudos to Adam, Beyolf, Vlad, Marius and all others...)
Pocco X3 NFC: Also running tests. Some users report that the recipient person has no sound. To be followed, but should eventually be fixed soon.
New upcoming devices: Brax3 / Volla Quintus: I assume VoLTE will be part of the port. No official info so far. Stay tuned....
I don't know about the other devices, especially regarding OnePlus - F(X)Tec - Pine64 and Asus supported smartphones. Maybe involved people will communicate in appropriate time.
So here are some clues if you need to choose a new device. But I strongly advise to wait for clear official announcements before to get a new one.