# Subscribe to User Defined Messages for a given Call SID.

**POST /2010-04-01/Accounts/{AccountSid}/Calls/{CallSid}/UserDefinedMessageSubscriptions.json**

Subscribe to User Defined Messages for a given Call SID.
Subscription for server-side application access to messages sent from the Voice SDK for an active Call.

## Servers
- https://api.twilio.com: https://api.twilio.com ()


## Authentication methods
- Account sid auth token


## Parameters

### Path parameters
- **AccountSid** (string)
  The SID of the [Account](https://www.twilio.com/docs/iam/api/account) that subscribed to the User Defined Messages.
- **CallSid** (string)
  The SID of the [Call](https://www.twilio.com/docs/voice/api/call-resource) the User Defined Messages subscription is associated with. This refers to the Call SID that is producing the user defined messages.


### Body: application/x-www-form-urlencoded (object)

- **Callback** (string(uri))
  The URL we should call using the `method` to send user defined events to your application. URLs must contain a valid hostname (underscores are not permitted).
- **IdempotencyKey** (string)
  A unique string value to identify API call. This should be a unique string value per API call and can be a randomly generated.
- **Method** (string(http-method))
  The HTTP method Twilio will use when requesting the above `Url`. Either `GET` or `POST`. Default is `POST`.


## Responses
### 201
Created
#### Headers
- **Access-Control-Allow-Origin** (string)
  Specify the origin(s) allowed to access the resource
- **Access-Control-Allow-Methods** (string)
  Specify the HTTP methods allowed when accessing the resource
- **Access-Control-Allow-Headers** (string)
  Specify the headers allowed when accessing the resource
- **Access-Control-Allow-Credentials** (boolean)
  Indicates whether the browser should include credentials
- **Access-Control-Expose-Headers** (string)
  Headers exposed to the client

#### Body: application/json (object)
- **account_sid** (string | null)
  The SID of the [Account](https://www.twilio.com/docs/iam/api/account) that subscribed to the User Defined Messages.
- **call_sid** (string | null)
  The SID of the [Call](https://www.twilio.com/docs/voice/api/call-resource) the User Defined Message Subscription is associated with. This refers to the Call SID that is producing the User Defined Messages.
- **sid** (string | null)
  The SID that uniquely identifies this User Defined Message Subscription.
- **date_created** (string(date-time-rfc-2822) | null)
  The date that this User Defined Message Subscription was created, given in RFC 2822 format.
- **uri** (string | null)
  The URI of the User Defined Message Subscription Resource, relative to `https://api.twilio.com`.


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