After upgrading my X23 to VollaOS 12 (12-20230911-STABLE-vidofnir), I followed the instructions at https://volla.online/en/faqs/ "How can I install another operating system?" (and, for lack of X23-specific instructions, ignoring the warning in the first line). It seems that Ubuntu is successfully installed on the SIM card. On reboot, it appears on the boot menu, and I can select it using the volume keys. However, the phone still boots into VollaOS.
Has anyone succeeded in booting Ubuntu as a second OS on the X23? Is it even possible yet?