# 21635: 'To' number cannot be a landline

Log Type: APPLICATION

Log Level: ERROR

## Description

This error occurs when you attempt to send an SMS message to a `To` number that is identified as a landline. In countries where Twilio does not attempt SMS delivery to landlines, the API rejects the request with this error.

### Possible causes

* The destination `To` number is a landline.
* You attempted to send an SMS to a landline in a country where Twilio blocks landline delivery attempts instead of sending the message to the carrier.

### Possible solutions

* Confirm that the destination number is not a landline by checking it with the [Lookup API](/docs/lookup/v2-api).
* Validate recipient numbers before sending so that your application only sends SMS to numbers that can receive text messages.
* If you still want to attempt delivery where supported, include the `ForceDelivery` API parameter in your Message request. This does not guarantee that the recipient can receive the message.
* Review how landline delivery is handled for the recipient's country before retrying. For landline numbers in the US, Canada, and the UK, Twilio may attempt delivery. In other countries, Twilio returns 21635.

#### Additional resources

* [Lookup API](/docs/lookup/v2-api)
* [Message resource](/docs/messaging/api/message-resource)
* [What happens when I attempt to send messages to a landline?](https://help.twilio.com/articles/223133647)