SMS message status | Polling or MessageEvent?

You can use the message-store event notification to avoid polling. But to check the message status, you still need to call the Get Message API to detect the status of the message. Here is a simple sample code in Node to check.

// Provided that you subscribe for this event filter
'/restapi/v1.0/account/~/extension/~/message-store?type=SMS&direction=Outbound'

// Then when you receive a new notification. Assumed that the msg is the notification payload

for (var message of msg.body.changes){
  for (var msgId of message.newMessageIds){
    await checkMessageStatus(msgId)
  }
}
async function checkMessageStatus(msgId){
  try{
    var resp = await platform.get(`/restapi/v1.0/account/~/extension/~/message-store/${msgId}`)
    var jsonObj = await resp.json()
    console.log("Message status: " + jsonObj.messageStatus)
  }catch(e){
    console.log(e.message)
  }
}