---
page_type: error
code: 21211
message: "21211: Invalid 'To' Phone Number"
level: ERROR
---

## Description

You attempted to initiate an outbound phone call or send a message, but the `To` phone number you supplied was not a valid phone number or was incorrectly formatted. Twilio accepts phone numbers in [E164 format](http://en.wikipedia.org/wiki/E.164): `[+] [country code] [subscriber number including area code]`.

### Possible Causes

- The formatting of the `To` number you supplied was invalid.
- You attempted to send a message or place a call from a Twilio number to itself (i.e. putting the same Twilio number in the `To` and `From` parameters).
- You attempted to send a message to Short Code.
- You attempted to send a message to an Alphanumeric Sender ID.
- You attempted to send a message to a `To` number with an invalid prefix using a Messaging Service.

### Possible Solutions

- Ensure you have formatted your phone numbers in [E164 format](http://en.wikipedia.org/wiki/E.164): `[+] [country code] [subscriber number including area code]`.
- Ensure you have used the correct [country calling code](https://en.wikipedia.org/wiki/List_of_country_calling_codes#Alphabetical_listing_by_country_or_region) for the phone number you are calling.
- Ensure you are not attempting to call or message from a Twilio number to itself.
- Ensure you are not attempting to send messages to Short Codes or Alphanumeric Sender IDs. These sender types cannot receive messages from Twilio numbers.
- Review the 'To' number for any typos or incorrect input methods.
- If you are using a third party integration, test your API request in the [Try it out](https://console.twilio.com/us1/develop/sms/try-it-out/send-an-sms) section of the Twilio console. If this is successful, you may need to contact the third party to ensure they are not making unexpected changes to your `To` parameter.
- When sending messages using a Messaging Service, instead of formatting a `To` number with an invalid prefix, remove the prefix and send directly to the `To` number using [E164 format](http://en.wikipedia.org/wiki/E.164): `[+] [country code] [subscriber number including area code]`. If the `To` number is able to accept RCS messages and there is an RCS Agent available in the Messaging Service Sender Pool, it will be selected to send to the given `To` number.