Fax Send with invalid number

Scenario: Testing invalid fax number response codes by faxing to the number 5

Expected: 400 status code with message telling me what field is bad and maybe a FaxResult type telling me InvalidOrMissingNumber (or 8)

Actual: Receive 500 response with no detail, occasionally a 400 status code is returned but with no detail describing what went wrong.

POST https://na.connect.etherfax.net/rest/3.0/api/outbox
{
"callerId": "1234567890",
"dialNumber": "5",
"faxImage": BASE_64_FILE_CONTENTS
}

There is inconsistency between the Http Status Code returned, and would be nice for some details regarding what went wrong.