transfer
Description:
Asynchronously joins all active and held calls, and disconnects from them.
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 transfer.end to get the result of the operation.
See transfer_sync for the synchronous, blocking version of this method.
Parameters:
| this | A ModemVoice. | 
| cancellable | A Cancellable or  | 
| callback | A TaskReadyCallback to call when the request is satisfied or  | 
| user_data | User data to pass to  |