A method of providing an internet protocol communications service to a mobile terminal, comprising the acts of: registering the mobile terminal with an Internet Protocol Multimedia Sub-system, IMS, for deploying multi-media communications sessions to the mobile terminal via a first mobile access network and a second mobile access network, the registration including authentication and authorisation of the mobile terminal, communicating a set-up request message from the mobile terminal acting as a calling terminal for requesting a bearer channel via said first mobile access network for communicating user data to and from a correspondent terminal acting as a called terminal to support a communications session with the correspondent terminal, communicating a session set-up message to said internet protocol multimedia sub-system from the first mobile access network, the session set-up message including an identifier of the mobile terminal and an identifier of the correspondent terminal, initiating the communications session between the mobile terminal and the correspondent terminal leaving the communications session unconfirmed with the mobile terminal, and consequent upon the mobile terminal roaming from the first mobile access network to the second mobile access network and requesting a communications bearer via the second mobile access network for continuing the communications session via the second mobile access network, establishing by the internet protocol multimedia sub-system the communications bearer via the second mobile access network, initiating by the internet protocol multimedia sub-system a session set up with the mobile terminal by acting as a termination for the communication session with the mobile terminal, comprising the step of communicating by the internet protocol multimedia sub-system a confirmation message to the mobile terminal via the second mobile access network to confirm that the communication session has been established to the mobile terminal, to provide the communications bearer via the second mobile access network for continuing the communications session with the mobile terminal via the second mobile access network, wherein the method further comprises the act of: establishing by the internet protocol multimedia sub-system the communications bearer via the second mobile access network, in response to at least one of a trigger from a radio communications layer of the second mobile access network, a trigger from a network communications layer of the mobile access, or in response to a session initiation message received by the internet protocol multimedia sub-system from the second mobile access network. registering the mobile terminal with an Internet Protocol Multimedia Sub-system, IMS, for deploying multi-media communications sessions to the mobile terminal via a first mobile access network and a second mobile access network, the registration including authentication and authorisation of the mobile terminal, communicating a set-up request message from the mobile terminal acting as a calling terminal for requesting a bearer channel via said first mobile access network for communicating user data to and from a correspondent terminal acting as a called terminal to support a communications session with the correspondent terminal, communicating a session set-up message to said internet protocol multimedia sub-system from the first mobile access network, the session set-up message including an identifier of the mobile terminal and an identifier of the correspondent terminal, initiating the communications session between the mobile terminal and the correspondent terminal leaving the communications session unconfirmed with the mobile terminal, and consequent upon the mobile terminal roaming from the first mobile access network to the second mobile access network and requesting a communications bearer via the second mobile access network for continuing the communications session via the second mobile access network, establishing by the internet protocol multimedia sub-system the communications bearer via the second mobile access network, initiating by the internet protocol multimedia sub-system a session set up with the mobile terminal by acting as a termination for the communication session with the mobile terminal, comprising the step of communicating by the internet protocol multimedia sub-system a confirmation message to the mobile terminal via the second mobile access network to confirm that the communication session has been established to the mobile terminal, to provide the communications bearer via the second mobile access network for continuing the communications session with the mobile terminal via the second mobile access network, wherein the method further comprises the act of: establishing by the internet protocol multimedia sub-system the communications bearer via the second mobile access network, in response to at least one of a trigger from a radio communications layer of the second mobile access network, a trigger from a network communications layer of the mobile access, or in response to a session initiation message received by the internet protocol multimedia sub-system from the second mobile access network.