Purism support just emailed me and let me know the following:
"I just word from our tech that where checking your account. And there was an issue with the phone number. And sadly that number is not recoverable.
What we can do is ship you a new SIM card free of charge with a new number. And have the same subscription."
This confirms my suspicion that there is nothing wrong with the Librem 5 itself, but rather with the service for this specific line. As such, I have zero problem reselling this Librem 5, as it is fully functional and will work with another line if you have the ability to get VoLTE working with your service.