---
page_type: error
code: 21635
message: "21635: 'To' number cannot be a landline"
level: ERROR
product: Programmable Messaging
---

## Description

You have attempted to send an SMS with a `To` number that is a landline.

### Possible Causes

The `To` number you provided is a landline number.

### Possible Solutions

- Confirm that the number you are sending to is not a landline, using the [Lookup API](/docs/lookup/v2-api).
- Include `ForceDelivery` [API parameter](/docs/messaging/api/message-resource#request-body-parameters) if you still want to send the message to this phone number. Please note that this does not guarantee the recipient number will be capable of receiving the message. Some international carriers may have a text-to-speech feature that converts the [SMS to a voice call](https://help.twilio.com/articles/223133647).