subscriber said data record indicating the codec that is to be used for the subsequent call call answer this support is explained in greater detail in the flow diagram of fig2 firstly a message sip invite is sent from the server s to the ip pstn gateway mgc this message could actually already contain the ip address of the client c because this is already known when the first http message arrives for a successful setup of the bearer rtp however at least the specification of the port of the client c and the list of the codecs which are supported by the client c are still missing at this time point and therefore the message sip invite does not contain an sdp data record or at least does not contain a complete sdp data record the ip pstn gateway mgc then uses a message 100 trying to show the server s that an attempt is being made to reach the subscriber b and performs the known setup of the bearer tdm in the network pstn in this context one pstn port leading into the network pstn and one rtp port leading into the network ip are occupied in the media gateway mg the signaling between the ip pstn gateway mgc and the subscriber b as per the protocol ss7 particularly the protocol.