Method for realizing Voice-over-IP communication sessions between a calling party and a called party using a telecommunications network comprising an access and/or aggregation network and an internet protocol backbone network, wherein each voice-over-IP communication session comprises a signaling session and a media flow session, wherein both the signaling session and the media flow session are routed via a first transport forwarding path network entity and a second transport forwarding path network entity, the calling party being associated or assigned to a first proxy call state control function entity or functionality and the called party being associated or assigned to a second proxy call state control function entity or functionality, and the media flow session comprising a first media flow in the direction from the calling party to the called party as well as a second media flow in the direction from the called party to the calling party, wherein the method comprises the following steps: -- in a first step, the media flow session of the voice-over-IP communication session is initiated by means of transmitting or exchanging a session description protocol offer message (145, 145') and a session description protocol answer message (146, 146') between the first and second proxy call state control function entity or functionality (131, 132) as part of the signaling session, and -- in a second step, after or during the first step, the first transport forwarding path network entity is configured, by a first software defined network controller entity or functionality, and involving at least one first configuration message transmitted to the first transport forwarding path network entity, such that the first transport forwarding path network entity implements modifications regarding endpoint address information of data packets of the first and/or second media flow (151, 152) on a session-by-session basis and dependent on at least a part of the content of the session description protocol offer message (145, 145') and the session description protocol answer message (146, 146'), wherein, during the first step, the calling party transmits the session description protocol offer message according to a first version thereof, comprising, as an endpoint address information of or regarding the media flow session, the address information of the calling party, and wherein the first proxy call state control function entity or functionality transmits the session description protocol offer message according to a second version thereof, comprising, as an endpoint address information of or regarding the media flow session, the address information of the first transport forwarding path network entity, and/or wherein, during the first step, the called party transmits the session description protocol answer message according to a first version thereof, comprising, as an endpoint address information of or regarding the media flow session, the address information of the called party, and wherein the second proxy call state control function entity or functionality transmits the session description protocol answer message according to a second version thereof, comprising, as an endpoint address information of or regarding the media flow session, the address information of the second transport forwarding path network entity. -- in a first step, the media flow session of the voice-over-IP communication session is initiated by means of transmitting or exchanging a session description protocol offer message (145, 145') and a session description protocol answer message (146, 146') between the first and second proxy call state control function entity or functionality (131, 132) as part of the signaling session, and -- in a second step, after or during the first step, the first transport forwarding path network entity is configured, by a first software defined network controller entity or functionality, and involving at least one first configuration message transmitted to the first transport forwarding path network entity, such that the first transport forwarding path network entity implements modifications regarding endpoint address information of data packets of the first and/or second media flow (151, 152) on a session-by-session basis and dependent on at least a part of the content of the session description protocol offer message (145, 145') and the session description protocol answer message (146, 146'), wherein, during the first step, the calling party transmits the session description protocol offer message according to a first version thereof, comprising, as an endpoint address information of or regarding the media flow session, the address information of the calling party, and wherein the first proxy call state control function entity or functionality transmits the session description protocol offer message according to a second version thereof, comprising, as an endpoint address information of or regarding the media flow session, the address information of the first transport forwarding path network entity, and/or wherein, during the first step, the called party transmits the session description protocol answer message according to a first version thereof, comprising, as an endpoint address information of or regarding the media flow session, the address information of the called party, and wherein the second proxy call state control function entity or functionality transmits the session description protocol answer message according to a second version thereof, comprising, as an endpoint address information of or regarding the media flow session, the address information of the second transport forwarding path network entity.