> 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/products/number-verification/reverse-sms.md).

# Reverse SMS

{% hint style="info" %}
**Backend API** · Early Access
{% endhint %}

Verify a claimed mobile phone number by having the user send an SMS from their device to an IDlayr-provided number. The carrier attributes the outbound SMS to a phone number, and IDlayr confirms whether it matches the claimed number. Unlike SMS OTP, the user doesn't type a code — the round trip is mobile-originated, not server-originated.

**No IDlayr SDK is required.** The verification is driven from your backend; the device-side step is your app launching the native SMS composer via a standard `sms:` URL scheme. Same product family as [Secure SNA](/products/number-verification/secure-sna.md) under the `/v1/number-verification/` URI namespace, but the trust anchor is the SMS channel, not the mobile-data path.

## What it does

* Verifies a claimed MSISDN by having the user send an outbound SMS from their device.
* The carrier's authoritative SMS-source record is the trust anchor.
* Returns a deterministic `match` boolean.

## What it doesn't do

* Doesn't prove the natural person using the device — like Secure SNA, this is a possession factor.
* Doesn't work on devices without SMS capability or where SMS sending is restricted.
* Doesn't silently dispatch — the user must tap Send in the native SMS composer. By design.

## When to use it

* As a **fallback** for Secure SNA when the device is on Wi-Fi only or the carrier doesn't support SNA.
* In **markets or line types** where SMS coverage is strong but SNA coverage is patchy.
* For **flows where cellular data isn't guaranteed** (rural areas, roaming, hotel Wi-Fi).

## In this section

* [**How it works**](/products/number-verification/reverse-sms/how-it-works.md) — the mobile-originated SMS flow.
* [**Prerequisites**](/products/number-verification/reverse-sms/prerequisites.md) — what you need before integrating.
* [**Integration**](/products/number-verification/reverse-sms/integration.md) — backend integration + the native SMS-composer launch.
* [**Testing in sandbox**](/products/number-verification/reverse-sms/testing.md) — sandbox MSISDN patterns.
* [**Endpoints**](/products/number-verification/reverse-sms/endpoints.md) — REST endpoint summary; full reference in API Reference.
* [**Callbacks**](/products/number-verification/reverse-sms/callbacks.md) — terminal-state callback payload, delivery, and retry.
* [**Error codes**](/products/number-verification/reverse-sms/error-codes.md) — HTTP errors and terminal-state codes specific to Reverse SMS.


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