Like others, I'm unable to apply for production due to the Webhook Subscriptions permission. My app's client ID is oQ7ud2BBRzGSbpixpBF1ZQ.
Using the RingCentral Phone for Desktop, you can dial or receive test calls, send and receive test SMS or Fax messages in your sandbox environment.
Download RingCentral Phone for Desktop:
Tip: switch to the "sandbox mode" before logging in the app:
7 People are following this question.