@Futura
If it doesn't work with Lineage OS, I think one of the following is probably happening:
The VoLTE patch for the Google Pixel 3a might not be in the Android image.
Lineage OS might not be picking it up.
Lineage OS supports VoLTE, but not VoLTE roaming. I read somewhere that the VoLTE patch only allows VoLTE roaming on the Google Pixel 3a.
The roaming setting might not be turned on in the Lineage OS settings.
The carrier doesn't support VoLTE roaming.
The carrier dropped support for the Google Pixel 3a.
I completely understand on the device switching and focusing on the Fairphone 5. No issues or worries there! I'm also on the market for a new phone as I'm certain this device is on borrowed time. I'm still looking to get a Fairphone 5 if I can. However, any progress made on the Google Pixel 3a would be good I think as it seems a number of users were using this phone in the US. So, until I can get a new phone, I'm going to be seeing what I can do with this one. Enjoy the Fairphone 5! I heard it is great with Ubuntu Touch on it!