> For the complete documentation index, see [llms.txt](https://docs.idlayr.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.idlayr.com/api-reference/reverse-sms/introduction.md).

# Introduction

The IDlayr Reverse SMS API verifies possession of a mobile phone number by having the end-user send (mobile-originate) a one-time SMS from the device being verified. There is no redirect and no inbound code to submit: the network attributes the originating message to a phone number and the platform resolves the match.

**Reverse SMS is a Backend API.** No IDlayr SDK is involved on the device. After your backend creates a Check, it returns the `receiver_phone_number` and `expected_sms_body` to your app, which opens the native SMS composer via the platform `sms:` URL scheme, for example:

```
sms:{receiver_phone_number}?body={url-encoded-expected_sms_body}
```

The user reviews and taps Send. The carrier attributes the source MSISDN; IDlayr resolves the match against the claimed phone number.

The flow is asynchronous:

1. **Create** the Check with the phone number you want to verify. The response carries the `receiver_phone_number` (the number the end-user must text) and `expected_sms_body` (the exact message body, including a one-time code).
2. **Hand off** `receiver_phone_number` and `expected_sms_body` from your backend to your app over your own application API.
3. **Send** — your app opens the native SMS composer pre-filled with both values; the end-user taps Send.
4. **Resolve** — once the platform receives the mobile-originated message the Check transitions to a terminal status and `match` reflects the result.

A webhook to `callback_url` (if you supply one) is fired when the Check reaches a terminal status. You can also poll `GET …/reverse-sms-checks/{check_id}`.

Requests authenticate with an OAuth2 Access Token in the `Authorization` header (`Bearer {access_token}`). Tokens are created using a project's `client_id` and `client_secret` and must carry the `reverse_sms` scope.


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://docs.idlayr.com/api-reference/reverse-sms/introduction.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.