I used the Teracube 2e (emerald) for a year: bought from Teracube last year with stock software; installed /e/OS Android R (Android 11) myself & never upgraded. VoLTE settings were never visible from the relevant Settings page and mobile calls cut off mobile data every single time.
Recently I installed /e/OS Android T (Android 13) on a OnePlus series phone and VoLTE looks abundantly supported (I can’t be sure since haven’t returned yet to my carrier country).
Therefore my best hope for “something to do” would be to install Android S (12) immediately, and then T if & when it comes out for the Teracube platform.