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

Log Type: APPLICATION

Log Level: ERROR

## Description

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`.

### Possible causes

* 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`).

### Possible solutions

* 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.