I would want to block a list of contacts such that I don't send them any SMS or call them nor can they
As per this ref : https://support.ringcentral.com/s/article/7251?language=en_US
The option to block SMS or MMS ONLY is currently NOT available
You can ask the back-end dev support team to do so
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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