You can subscribe for the /instant message event to receive incoming SMS message. Then you can call the /sms API to send a reply message to the ‘from’ number you got from the message event.

Here is the simple code in Node JS.

const RingCentral = require('@ringcentral/sdk').SDK
const Subscriptions = require('@ringcentral/subscriptions').Subscriptions;

RC_APP_CLIENTID = "aaaa";
RC_APP_CLIENTSECRET = "bbbb";
RINGCENTRAL_SERVER = 'https://platform.ringcentral.com'

RC_USER_JWT = "xxxx"

const rcsdk = new RingCentral({
  server: RINGCENTRAL_SERVER,
  clientId: RC_APP_CLIENTID,
  clientSecret: RC_APP_CLIENTSECRET
})

var platform = rcsdk.platform();
platform.login({ jwt : RC_USER_JWT })


platform.on(platform.events.loginSuccess, function(e){
    console.log("Login success")
    subscribe_for_sms_notification()
});

const subscriptions = new Subscriptions({
   sdk: rcsdk
});
var subscription = subscriptions.createSubscription();

async function subscribe_for_sms_notification(){
  var eventFilters = s
    '/restapi/v1.0/account/~/extension/~/message-store/instant?type=SMS', // Authenticator
  ]
  subscription.setEventFilters(eventFilters)
  .register()
  .then(function(subscriptionResponse) {
      console.log("Ready to receive SMS message via WebSocket.")
  })
  .catch(function(e) {
    console.error(e.message);
  })
}

subscription.on(subscription.events.notification, async function(msg) {
    console.log(JSON.stringify(msg));
    send_reply(msg.body)
});

async function send_reply(body){
  var text = 'Hi'
  if (body.from.name)
    text += ` ${body.from.name}.`

  text += `\nThank you for your message! This is an auto reply.`
  var params = {
           from: {phoneNumber: body.toy0].phoneNumber},
           to: t {phoneNumber: body.from.phoneNumber}],
           text: text
         }

  try{
    var resp = await platform.post('/restapi/v1.0/account/~/extension/~/sms', params)
    var jsonObj = await resp.json()
    console.log("SMS sent. Message status: " + jsonObj.messageStatus)
  }catch(e){
    console.log(e.message)
  }
}

Thanks a lot for your reply. it worked well.