Whilst the IMS is designed as a standalone system to provide multimedia services to its users, it can also be used to provide additional functionalities within existing telephony networks. During the initial phase of IMS deployment mobile network users will make use mainly of the CS domain of the mobile networks for audio sessions. In such CS calls the IMS is not required for provision of the basic service – i.e., the signalling and media connection between the two users, as well as the QoS.
But also such CS calls should benefit from the flexible and broad service provisioning capabilities of IMS. New work has been started by 3GPP recently, to enable such so-called "combinational services". As the work is still in progress at the time of writing, only the basic principles can be shown here.
In order to enable such a service combination, the two UEs first need to become aware of the supported capabilities at the other end. Section 3.18.2 shows how IMS-related capabilities can be exchanged after a CS call has been established between two users. In Section 3.18.3 it is shown how, based on that capability exchange, an IMS service can be invoked that enhances the ongoing CS call.
In the following example we will assume that two users are connected over the CS domain – i.e., they have a CS connection between them. In this example Tobias called his sister via her international ...