An apparatus comprising: means for establishing an Internet protocol, IP, flow with a network, wherein the establishing includes assigning a reservation label to the IP flow; means for preconfiguring, by negotiation with the network prior to establishing a voice call session associated with the IP flow, a network resource first instance that achieves a quality of service, QoS, meeting a QoS profile for the IP flow, and is available for subsequent reservation by sending the network a request to turn ON the reservation label; means for sending an invite message to another terminal, to initiate the voice call session; means for, prior to receiving a response from the another terminal to the invite message, sending to the network the request to turn ON the reservation label, and receiving from the network a reservation accept indicating network acceptance of the request to turn ON the reservation label; means for receiving, via the network, the another terminal's response to the invite message, wherein the another terminal's response identifies a media format supported by the another terminal; means for determining whether the media format identified by the another terminal's response is compatible with the QoS profile, and means for, if the media format identified by the another terminal's response is determined compatible with the QoS profile, leaving the QoS profile unchanged, and means for, if the media format identified by the another terminal's response is determined not compatible with the QoS profile, then negotiating, with the network, changes in parameters of the QoS profile for compatibility with the media format identified by the another terminal's response, and modifying the network resource instance corresponding to the ON reservation label, if required to meet the changes in parameters of the QoS profile; and means for establishing the voice call session initiated by the invite message, in association with the ON reservation label. means for establishing an Internet protocol, IP, flow with a network, wherein the establishing includes assigning a reservation label to the IP flow; means for preconfiguring, by negotiation with the network prior to establishing a voice call session associated with the IP flow, a network resource first instance that achieves a quality of service, QoS, meeting a QoS profile for the IP flow, and is available for subsequent reservation by sending the network a request to turn ON the reservation label; means for sending an invite message to another terminal, to initiate the voice call session; means for, prior to receiving a response from the another terminal to the invite message, sending to the network the request to turn ON the reservation label, and receiving from the network a reservation accept indicating network acceptance of the request to turn ON the reservation label; means for receiving, via the network, the another terminal's response to the invite message, wherein the another terminal's response identifies a media format supported by the another terminal; means for determining whether the media format identified by the another terminal's response is compatible with the QoS profile, and means for, if the media format identified by the another terminal's response is determined compatible with the QoS profile, leaving the QoS profile unchanged, and means for, if the media format identified by the another terminal's response is determined not compatible with the QoS profile, then negotiating, with the network, changes in parameters of the QoS profile for compatibility with the media format identified by the another terminal's response, and modifying the network resource instance corresponding to the ON reservation label, if required to meet the changes in parameters of the QoS profile; and means for establishing the voice call session initiated by the invite message, in association with the ON reservation label.