To send text messages or Alimtalk in South Korea you must **register the sending number in advance**. This is not a Sendgo policy — it is a legal requirement (전기통신사업법 §84-2, prohibition on caller ID spoofing).

This step is the usual bottleneck. Document review takes time, and most teams start it only after the code is written. **Start it first.**

## Two kinds of sender key

| Key | Identifies | Used by |
| --- | --- | --- |
| `senderKey` | A registered **SMS sending number** | SMS · LMS · MMS, and Alimtalk SMS fallback |
| `kakaoSenderKey` | A connected **KakaoTalk channel** (sender profile) | Alimtalk, Brand Message |

Alimtalk alone needs only `kakaoSenderKey`, but the moment you enable [SMS fallback](/en/cookbook/sms-fallback) you also need `senderKey`. In practice almost everyone enables fallback, so **prepare both**.

## Registering an SMS sending number

1. Sendgo console → **Sender numbers** → Register
2. Enter the number and the registered holder's details
3. Submit documentation
   - Proof of subscription showing the number belongs to the applicant
   - Business registration certificate, for a corporate holder
   - Letter of delegation, if the number is in someone else's name
4. Wait for review

**Where this usually fails**

- The name on the proof does not match the business details on the account. Common when registering a branch or subsidiary number under a head-office account.
- The document is old. Reissue it — recent dates are expected.
- Representative numbers (1588, 1600 and similar) may need extra line-ownership verification.

## Connecting a Kakao sender profile

Alimtalk needs a **KakaoTalk channel**, separate from the SMS number.

1. Create a KakaoTalk channel at [Kakao Business](https://business.kakao.com)
2. Convert it to a **business channel** (requires business verification)
3. Sendgo console → **Kakao sender profile** → Register
4. Enter the channel search ID and verify via the administrator's phone
5. Store the issued `kakaoSenderKey`

```bash
SENDGO_KAKAO_SENDER_KEY=your_kakao_sender_key
SENDGO_SMS_SENDER_KEY=your_sms_sender_key
```

**Where this usually fails**

- **A regular channel will not work.** Without the business conversion you cannot create a sender profile.
- You must verify with a Kakao account that administers the channel.
- The channel search ID is the `@`-prefixed identifier, not the display name.

## Sending before registration completes

An unregistered number fails **at send time, not at signup**.

| Code | Cause |
| --- | --- |
| `INVALID_KAKAO_SENDER_KEY` | Sender profile key does not exist or is not owned by this account |
| `SENDER_NOT_REGISTERED` family | SMS sending number unregistered or still pending |

Check the console shows **approved**, not "under review". Pending numbers do not send.

## Next

- [Send your first Kakao Alimtalk in five minutes](/en/cookbook/quickstart)
- [Registering and getting an Alimtalk template approved](/en/cookbook/alimtalk-template)