join_multiparty
Description:
[ Version ( since = "1.12" ) ]
public async bool join_multiparty (Cancellable? cancellable) throws Error
  public async bool join_multiparty (Cancellable? cancellable) throws Error
Synchronously requests to join this call into a multiparty call.
When the operation is finished, callback will be invoked in the thread-default main loop of the thread you are calling this
      method from. You can then call join_multiparty.end to get the result of the operation.
See join_multiparty_sync for the synchronous, blocking version of this method.
Parameters:
| this | A Call. | 
| cancellable | A Cancellable or  | 
| callback | A TaskReadyCallback to call when the request is satisfied or  | 
| user_data | User data to pass to  |