SMS-RC-503 while sending A2P message.

If you see the error code details: https://ringcentral-api-docs.readthedocs.io/en/latest/messaging/sms/high-volume/handling-errors/

It's written as "RingCentral provisioning error. Phone number is incorrectly provisioned by RingCentral in upstream. "

You can contact Dev support team to fix

Have you used this form to request for A2P number provisioning? If not, please do so. Otherwise, click here to create a support ticket and provide the phone number and your account id or name.