# 21665: Invalid 'FallbackFrom' sender

Log Type: APPLICATION

Log Level: ERROR

## Description

The `FallbackFrom` parameter must be a valid SMS or MMS sender. Aceptable values include an E.164 phone number, an alphanumeric sender ID, or a short code that exist in your Twilio account.

### Possible causes

* `FallbackFrom` contains a phone number not in E.164 format (e.g., `15105550100` instead of `+15105550100`).
* `FallbackFrom` contains a phone number in an unsupported format (e.g., with spaces or parentheses).
* `FallbackFrom` contains a phone number that is not owned by the Twilio Account
* `FallbackFrom` contains an RCS Sender format (e.g., `rcs:sender_id`).

### Possible solutions

* Use a valid SMS/MMS sender format: an E.164 phone number (e.g., `+15105550100`), alphanumeric sender ID, or short code.
* Do not use an RCS Sender format in `FallbackFrom`. The RCS Sender for the message is selected automatically from the Messaging Service sender pool.
* Ensure the Twilio account being used owns the phone number.