If sending a fax fails, the fax message status will be 'SendingFailed'. The failed fax message should be still in the user message store. Make sure that you authenticate your app with the same user, or if you read the message store of other users (as a super admin user) you need to specify the correct user id in the API path.

What is the best way to go about checking if messages were sent, or failed? I am using rest API 

After sending a (Fax) message, grab the message id from the API response and call this API to check the status.

How long after should I check the status? Would it take an hour, minutes or seconds to know if the fax failed?

How about for a busy line, say the fax line I am sending to is busy. What would show ?

The system will retry sending several times. You can set a timer to check every 20 secs until you get it Sent or SendingFailed.

But if you don’t want to poll, you can subscribe for the fax event notification and every time you get an event, you can check the status of the fax.

eventFilters: s
    '/restapi/v1.0/account/~/extension/~/message-store?type=Fax&direction=Outbound'
]

// Parse the event and check the fax status
...
var messageIds =  ]
if (jsonObj.body.changesg0].newCount > 0){
    messageIds = jsonObj.body.changesg0].newMessageIds
    checkFaxStatus(jsonObj.body.changesg0].newMessageIdsI0])
}else if (jsonObj.body.changesg0].updatedCount > 0){
    if (messageIds.length > 0){
        checkFaxStatus(messageIdsI0])
    }
}


//
async function checkFaxStatus(id){
  try{
    var resp = await platform.get('/restapi/v1.0/account/~/extension/~/message-store/' + id)
    var jsonObj = await resp.json()
    console.log(jsonObj)
  }catch(e){
    console.log(e.message)
  }
}

I have one last question, when making these calls, would I be able to see responses from faxes sent through our email via RingCentral?

Would it make more sense to call to the “Call-Log”? 

What is the difference between what the call log shows and the message store?

Would it be possible to use Analytics API to see failed fax?

I don’t think so. But you can test yourself by sending a fax and check the message store API.

The call log gives you the fax call metadata. The message store gives you the fax message details with attachments, cover page index and text.

No, currently the Analytics data do not include Fax call data.