If SIM card is enabled before phone is placed into Airplane mode, then voicemail notifications occur after a reboot as if rebooting in Airplane mode results in phone making a brief cellular connection immediately after reboot before phone is unlocked. This happens evenwhen WiFi Calling is disabled and even if WiFi is not connected in Airplane mode at all.
If SIM card is disabled before the phone is placed into Airplane mode and rebooted, then voicemail notifications don't come up immediately after that reboot.
That makes me think that rebooting in Airplane mode does result in a brief connection to cellular service after reboot, but I don't know for sure.