21664: 'FallbackFrom' cannot be used with a 'From' sender

The FallbackFrom parameter cannot be used with a From parameter that specifies a phone number or sender. To use FallbackFrom, the RCS Sender must be selected by the Messaging Service — not specified directly in From.

• Both FallbackFrom and From are specified in the same request, where From contains an E.164 phone number (e.g., +15105550100).
• Both FallbackFrom and From are specified in the same request, where From contains an RCS Sender (e.g., rcs:sender_id).

• To use RCS with a specific SMS/MMS fallback number: Remove From and use MessagingServiceSid with FallbackFrom.
• To send SMS/MMS from a specific number without RCS: Remove FallbackFrom and use From only.
• Using a Messaging Service SID as the From value is valid and does not conflict with FallbackFrom, but it's recommended to use the MessagingServiceSid parameter.