The error message is clear. You try to send SMS from a number which the user (who is authenticated using your app) is not the owner of that phone number.

Having a look at your account settings, the "from" phone number is your company main company number, and the extension 333 is assigned to be the owner of that number. Thus, you have to authenticate the app with the user (Ext. 333) login credentials (NOT the 101).

Alternatively, you can reassign the extension who supposed to handle incoming and SMS for your main company number and any other company numbers, via the Auto-receptionist -> General settings

Thanks for your help,

Our issue got resolved and now we are able to send and receive messages for RingCentral.

We just want you to help us with the Webhook also. Our Webhook got blacklisted after a few days. And to prevent this, we have done the coding also that use to call the API endpoint every 10 mins, We had read the documentation related to the webhook blacklisted. But still, Our webhook response gets terminated.

Can you please help us, so that our webhook should not get blacklisted?

Unfortunately, with that I cannot help much. If your webhook server is unreachable for any reason, the address will be blacklisted.

You can submit a dev support ticket to get help.