The account number setup varies depending on the customer.

Generally the acct_number_internal sent by HQ with the request will have the account in a format the FI is comfortable with displaying to the end user. It could be either masked or unmasked.

If you need the full account number there are acct_number_internal_unmasked and acct_number_external_unmasked.

A HADE with the name "AccountNumber" is rare.