> ## Documentation Index
> Fetch the complete documentation index at: https://docs.hologram.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Troubleshooting undelivered SMS messages

> Diagnose SMS delivery issues for messages sent to or from a Hologram device.

## Understand SMS delivery statuses

* **Sending from a Hologram device:** "Delivered" means Hologram delivered the SMS to the receiver's network; last-mile delivery to the device is not guaranteed.
* **Sending to a Hologram device:** "Delivered" means the sender's network delivered to the network your device is attached to; last-mile depends on that network and device state.

<Note>
  **Note:** Charges apply for a successful send from Hologram, even if the end device does not receive the SMS.
</Note>

## Common issues

### Carrier blocks international SMS

Some carriers block international SMS by default (Hologram uses global, non-domestic numbers). Ask the carrier to allow messages from international numbers (for example, +882 or +999).

### Cat-M1 SMS maturity

On some networks, SMS over Cat-M1 may be delayed or require both sender and receiver to be on the same Cat-M1 network.

### Incorrect SMSC routing

Misconfigured SMSC mappings on either network can prevent delivery. These mappings are controlled by carriers.

## Additional troubleshooting

<Steps>
  <Step title="Check Hologram status">
    Confirm there are no incidents affecting SMS or API.

    <Card title="Status page" href="https://status.hologram.io/" horizontal />
  </Step>

  <Step title="Verify device state and signal">
    Ensure both devices are powered and have adequate coverage.
  </Step>

  <Step title="Test with a different receiver">
    If sending from a Hologram device, test to another device (ideally on the same carrier) and confirm it receives SMS.
  </Step>

  <Step title="Read SMS via AT commands">
    Use `AT+CMGL` or the device's documented command set to read stored SMS.
  </Step>
</Steps>

## Related guides

<Card title="AT command reference and troubleshooting cellular modules" href="/guides/troubleshooting/at-command-reference-and-troubleshooting-cellular-modules" horizontal>
  AT command reference and troubleshooting cellular modules
</Card>