So an update. I contacted WhatsApp support and they finally got back to me. Essentially I explained my issue but also mentioned I was using GrapheneOS on a Pixel 6 Pro. Their reply included "Custom ROMs...aren't supported":
Your attempt to register your phone number failed because you appear to be using an unofficial version of the WhatsApp app. We don’t support unofficial apps because they put your privacy and security at risk. They also put your data and device at the risk of malware.
Custom ROMs and rooted phones also aren't supported by WhatsApp, learn more in this article.
What to do next
- Save your chat history.
- Uninstall the unofficial app.
- Download WhatsApp or the WhatsApp Business app from official app stores or our website.
- Register your phone number.
If you see an error that the app isn’t available in your country, visit this page to download WhatsApp.
Need more help?