Can you give me an example of a callback event?

Here's an example of a callback event, which would be POSTed to your callback URL.

In this case, it's a SENT status update on an outbound fax:

POST /callback.php?type=outbound HTTP/1.1
Host: your.callbackdomain.here
Accept: */*
Expect: 100-continue
Content-Type: multipart/form-data; boundary=----------------------------e2bdba1d3cf0
Content-Length: 520
X-Forwarded-For: 52.200.252.64

------------------------------e2bdba1d3cf0
Content-Disposition: form-data; name="json"

{"Transaction":{"Guid":"[Fax GUID]","To":"14154847140","From":"14154847140","IsInbound":false,"IsDraft":false,"TypeCode":"F","StatusCode":"S","ErrorCode":null,"CreatedAt":"1505933982","UpdatedAt":"1505934022","Uri":"\/v1\/Accounts\/[Your Account GUID]\/Transmissions\/[Fax GUID]","NumPagesBilled":1}}
------------------------------e2bdba1d3cf0--

See the article on sending a fax using the Dropbox Fax API for more information.

Here is an example of a callback process for an outbound fax with an error (in this case, it's a bad Recipient Fax Number):

{
   "Transaction":{
      "Guid":"[Fax GUID]",
      "To":"1214155555555",
      "From":"14155555555",
      "IsInbound":false,
      "IsDraft":false,
      "TypeCode":"F",
      "StatusCode":"E",
      "ErrorCode":"A",
      "CreatedAt":"1475766509",
      "UpdatedAt":"1475766550",
      "Uri":"\\/v1\\/Accounts\\/[Your Account GUID]\\/Transmissions\\/[Fax GUID]",
      "NumPagesBilled":0
   }
} 

NOTE: we know of an issue where if you have an Account Callback URL profiled in the hellosign.com API Settings page, you'll receive Dropbox Fax events similar to this:

{
   "event":{
      "event_type":"transmission_disconnected",
      "event_time":"1475766511",
      "event_hash":"9a306f1874007d9f40f946e21480dcb9a1688516776cbea6f471c4c1e0db1502",
      "event_metadata":{
         "related_signature_id":null,
         "reported_for_account_id":"[Your Account GUID]",
         "reported_for_app_id":null,
         "event_message":null
      }
   },
   "account_guid":"[Your Account GUID]",
   "client_id":null,
   "transmission":{
      "test_mode":false,
      "title":"fillable w9 test doc",
      "original_title":"fillable w9 test doc",
      "subject":null,
      "message":null,
      "metadata":{

      },
      "from":"1415555555",
      "transmissions":[
         {
            "status_code":"error_disconnected",
            "recipient":"12141555555"
         }
      ],
      "transmission_id":"[Transmission GUID - Different than Fax GUID]",
      "final_copy_uri":"\\/v3\\/transmission\\/final_copy\\/[Transmission GUID - Different than Fax GUID]",
      "files_url":"https:\\/\\/api.hellofax.com\\/v3\\/transmission\\/files\\/[Transmission GUID - Different than Fax GUID]"
   }
}

You can choose to parse or ignore these events as needed - they're not strictly required. The Dropbox Fax callback Transaction is the one that's explicitly regarding the Dropbox Fax faxing attempt.