Welcome to the RingCentral Community

It must be your SMS app on the phone or your carrier restriction which strip of the brackets.

Here is the screenshot of what I send from RC API to my cell number.

And this is the code where I call to send the message

async function send_sms(fromNumber){
  try{
    var resp = await platform.post('/restapi/v1.0/account/~/extension/~/sms', {
             from: {phoneNumber: fromNumber},
             to: [{phoneNumber: RECIPIENT}],
             text: '{"key":"value", "array":[{"key1":"value1","key2":1},{"key1":"value1","key2":1}]}'
        })
    var jsonObj = await resp.json()
    console.log("SMS sent. Message status: " + jsonObj.messageStatu
  }catch(e){
    console.log(e.message)
  }
}