ETS 300 user-side specifications describe interactions between a user (an application) and the network from the user's point of view. In an ETS 300 configuration, the network performs the actual transfer operation (connects the two remote parties together). At least one of the parties in the transfer must have invoked the hold service.
The application's role in a call transfer is to request the network to perform the transfer using the explicit call transfer service. As shown in the following illustrations, once the transfer is complete, affected connection IDs and B channels over the S/T reference point are cleared so the local application is no longer involved in the call.
Note: After the ECT service is invoked, the user side cannot recover the call from the network.
When an explicit call transfer succeeds, the network informs the remote parties of the transfer using a notify transfer operation.
The application is responsible for assuring that the joined parties are compatible. For example, both parties should be carrying the same type of service (data or voice).
To perform a call transfer, an ETS 300 application invokes the explicit call transfer (ECT) service. This service allows an application serving two calls, one of which is in the auxiliary HELD state, to request the network to join the two remote calls together and drop the local connection.