# Public Switched Telephone Network (PSTN)

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The Video SDK call out feature allows users to join Video SDK sessions completely over the phone or use their phone as the audio connection to the session while viewing the Video on Web. This is useful when:

-   You do not have a microphone or speaker on your computer.
-   You do not have an iOS or Android smartphone.
-   You cannot connect to a network for a video session with computer audio (VoIP).

See [Customize audio conferencing](/docs/video-sdk/account/#customize-audio-conferencing) for details.

## Prerequisites

-   [Video SDK Account](https://zoom.us/pricing/developer)
-   [Audio Conferencing Plan](https://zoom.us/billing)

## Get instance

To use PSTN, you must first get an instance of the `ZoomVideoSdkPhoneHelper` object.

```dart
ZoomVideoSdkPhoneHelper phoneHelper = zoom.phoneHelper;
```

## Check whether the account supports PSTN

Use `phoneHelper.isSupportPhoneFeature()` to check whether the account supports PSTN.

```dart
await zoom.phoneHelper.isSupportPhoneFeature();
```

If the account supports the feature, you can continue. If not, you may want to display a message to the user that this feature is not supported.

## Make a call

Use `phoneHelper.inviteByPhone()` to make a call.

-   Provide the name if the user will join completely by phone.
-   If the user is already connected to the session on the web, but wants to use their phone for the audio portion, the name will default to the name provided in the `client.join()` function.

```dart
await zoom.phoneHelper.inviteByPhone("<country code>", "<phoneNumber>", "<name>");
```

## Receive events

Use the `onInviteByPhoneStatus` event listener to find out about the status of the call, for example, ringing, accepted, hangup, etc.

```dart
final inviteByPhoneStatusListener =
  eventListener.addListener(EventType.onInviteByPhoneStatus, (data) async {
    data = data as Map;
    debugPrint(
      "onInviteByPhoneStatus: status: ${data['status']}, reason: ${data['reason']}");
  });
```