---
"@context": https://schema.org
"@type": TechArticle
"@id": https://www.twilio.com/docs/api/errors/21666#article
headline: "21666: 'FallbackFrom' requires MessagingServiceSid"
url: https://www.twilio.com/docs/api/errors/21666
inLanguage: en
dateModified: 2026-04-23T04:41:07.000Z
author:
  "@type": Organization
  name: Twilio Developer Education Team
publisher:
  "@type": Organization
  name: Twilio
---

# 21666: 'FallbackFrom' requires MessagingServiceSid

Log Type: APPLICATION

Log Level: ERROR

## Description

When you use `FallbackFrom`, also provide `MessagingServiceSid`. `FallbackFrom` sets the SMS or MMS sender Twilio uses if an RCS message can't be delivered over RCS. This parameter works only with a Messaging Service.

### Possible causes

* Your request included `FallbackFrom` but didn't include `MessagingServiceSid`.
* Your request used `FallbackFrom` with `From` instead of sending through a Messaging Service.
* Your application attempted to set an RCS fallback sender without routing the message through a Messaging Service.

### Possible solutions

* Add `MessagingServiceSid` to the request when you use `FallbackFrom`.
* Send the message through a Messaging Service that you use for RCS messaging.
* If you don't need RCS fallback, remove `FallbackFrom` and send with `From` or `MessagingServiceSid` alone.
* Verify that the fallback sender you specify in `FallbackFrom` is an SMS- or MMS-capable sender that belongs to the same Account SID.

#### Additional resources

* [Messages resource](/docs/messaging/api/message-resource)
* [Messaging Services](/docs/messaging/services)
* [Send and receive RCS messages](/docs/rcs/send-an-rcs-message)