21665: Invalid 'FallbackFrom' sender

The FallbackFrom parameter must be a valid SMS or MMS sender. Use a Twilio phone number in E.164 format, an alphanumeric sender ID, or a short code that belongs to the Account SID creating the message. FallbackFrom is used only for RCS fallback with a MessagingServiceSid.

• FallbackFrom contains a phone number that isn't in E.164 format, such as 15105550100 instead of +15105550100.
• FallbackFrom contains unsupported phone number formatting, such as spaces or parentheses.
• FallbackFrom contains a phone number, alphanumeric sender ID, or short code that isn't hosted within Twilio or doesn't belong to the same Account SID that is creating the message.
• FallbackFrom uses an RCS sender value such as rcs:sender_id instead of an SMS or MMS sender.

• Pass a valid SMS or MMS sender in FallbackFrom. Use a Twilio phone number in E.164 format, an alphanumeric sender ID, or a short code.
• Remove extra formatting characters from phone numbers before sending the request.
• Use a sender that belongs to the same Account SID that is creating the message. The sender doesn't need to be in the Messaging Service sender pool.
• Do not pass an RCS sender in FallbackFrom. Let the Messaging Service select the RCS sender automatically and use FallbackFrom only for the SMS or MMS fallback sender.

• Messages resource
• Messaging Services
• What is E.164?