If your phone number is SMS-enabled by RingCentral, you'll need to contact RingCentral and ask them to opt out the number from SMS.

First, submit an SMS Opt Out Request to RingCentral's Salesforce/Tier1 Support. You will be instructed what to do from there. If you have questions about how to submit a ticket, contact your RingCentral Account Representative.
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Once the opt out process is complete, RingCentral will send you an Letter of Authorization (LOA). Send this information to TextUs Support so we can enable SMS for your phone numbers through our provider and create your TextUs accounts.