Eventually, I realized where the problem was, it seems that Meta doesn't update its documentation quickly enough. The On-Premises API is sunsetting, so we can't use the documentation from the certificate. We only need to register the number using the endpoint "https://graph.facebook.com/v20.0/[phone-number-id]/register" without associating any certificate. Here is the documentation: https://graph.facebook.com/v20.0/[phone-number-id]/register.