I now have successfully placed VoWiFi calls (and sent SMSoWiFi texts) in airplane mode with some caveats:
Working phones: Pixel 7 Pro cheetah and a brax3 both with iodé 6.9 with a T-Mobile USA MVNO (US Mobile) SIM card.
A Verizon USA MVNO (US Mobile) SIM card does not succeed in any device, and a Pixel 3a XL bonito doesn’t work for VoWiFi calls even with the T-Mobile network SIM card.
Here is the same info, but as a reply to the gitlab issue: Placing VoWiFi calls fails without cellular connectivity (#38) · Issues · ota / issue-tracker · GitLab