# 21665: Invalid 'FallbackFrom' sender

Log Type: APPLICATION

Log Level: ERROR

## Description

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`.

### Possible causes

* `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.

### Possible solutions

* 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.

#### Additional resources

* [Messages resource](/docs/messaging/api/message-resource)
* [Messaging Services](/docs/messaging/services)
* [What is E.164?](/docs/glossary/what-e164)