When you send a fax using the API, the response will contain the 'messageId'. You can also look this up in the 'message-store' or 'call-log' APIs.
Then you can use the 'messageId' to associate the message in the 'message-store' and 'call-log' endpoints. The message store entry includes information on the canonical message while the call log record includes information on a specific transmission attempt. The call log entry has a 'message' property with both 'message.id' and 'message.url' properties for cross referencing.