I have recently added the "Edit Messages" to my application scopes, but I am getting an internal service error on every request.
I see failure on PUT call to the /message-store API. But I don't know what was in the request body. You should submit a dev support ticket to get help with this.
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:
9 People are following this question.