I've spent about 10 minutes looking at articles about RC Classic messages and I can't find it.
Yes, there is no official document regarding this and haven't found anywhere mentioning this.
However, officially there is another way mentioned to post code:
Hope this help others in the community
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:
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