21667: 'FallbackFrom' requires the Messaging Service to have an RCS Sender

FallbackFrom requires the Messaging Service to have at least one RCS Sender in its sender pool. FallbackFrom identifies the SMS or MMS sender that Twilio uses if RCS delivery fails.

• The Messaging Service referenced by MessagingServiceSid has no RCS Sender in its sender pool.
• An incorrect MessagingServiceSid was provided (e.g., a Messaging Service configured for SMS-only use).
• The RCS Sender was removed from the Messaging Service sender pool after the integration was set up.

• Add an RCS Sender to the Messaging Service sender pool in the Twilio Console.
• Verify that the correct MessagingServiceSid is being used.
• If RCS is not needed, remove FallbackFrom and send using the Messaging Service or From parameter without it.