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It's the RingCentral filter that marks the content as spam. It's likely because the text content does not match the criteria provided while your company register for TCR campaign registration, or the message contains keywords that forbidden etc. The reason RingCentral also filters your message is that it helps you avoid paying for sending spam messages which potentially will be blocked by mobile carriers if the message is sent out from RingCentral to the carriers.

You should submit a customer support ticket and provide the account info, your phone number, a few recipient numbers (that failed) and the actual text message so they can help solving your problem.