Thanks so much for sharing this solution!

Meta’s documentation doesn't make this clear at all, and the error message 133010: The account is not registered is super misleading.

So just to make it crystal clear for anyone else who finds this:

For future readers:

Having your number “verified” in Business Manager does NOT mean it’s registered in the API.

You must call:

POST https://graph.facebook.com/v18.0/\<PHONE_NUMBER_ID>/register

with a 6-digit PIN and your access_token.

If you don’t do this, you’ll keep getting the dreaded 133010 error forever.

Thanks again — you saved my sanity (and possibly what's left of my weekend 😅).