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Still can't understand the logic behind all your answers, sometimes you point the user to the dev team and sometimes you provide them with a piece of code to check is they have access to some specific options ....

It will make sense if you put this function in your GUI interface right ?

Use this piece of code to check if the number you use to send SMS has SMS feature. If the number has the SmsSender then something is wrong with the account/extension setting => open a support ticket with your account/extension info. Otherwise, choose a number that has the SMS feature

function check_extensions_phone_number() {
  platform.get('/account/~/extension/~/phone-number')
    .then(function(resp){
      var jsonObj = resp.json()
      for (var record of jsonObj.records){
        if (record.usageType == "DirectNumber"){
          console.log("DirectNumber Found. Check feature")
          console.log(record.features)
          console.log(record.phoneNumber)
          for (var feature of record.features){
            if (feature == "SmsSender"){
              console.log("This number supports SMS: " + record.phoneNumber)
            }
          }
        }
      }
    })
    .catch(function(resp){
      console.log("Something went wrong.")
    })
}

HTTP POST https://platform.ringcentral.com/restapi/v1.0/account/~/extension/~/sms

What API did you call?