ormation. After exchanging the SDP Offer and the SDP Answer through INVITE and 183 Session Progress messages, the PCF is triggered to start establishing the QoS flows in the 5G network for the media flow. This is not shown in the following diagram and will be analyzed in the following section. The next message that is sent is the Provisional Response Acknowledgement (PRACK), which is a confirmation message to tell the called party that the calling party received the previous 183 Session Progress message. In response to that, the called party sends a 200 OK message to confirm that it received the PRACK. As mentioned before, the 5G networks for the calling and called party will be busy setting up the resources required for the media flow, and there is a technique used in VoLTE and VoNR called preconditions. Fundamentally, a precondition is an agreement between the calling and called party devices that before the called party is even alerted that somebody is ringing them, the calling and called party start to establish their media bearers. That's exactly what is happening in this flow. The called party is not ringing at this point. The idea is to make sure that the correct QoS for the media flow resources and additional media QoS flows are in place in the respective 5G networks for the calling and the called party before the called party is alerted. So, UPDATE (SDP Offer) and 200 OK (SDP Answer) in the signaling exchange are used to set up the preconditions and establish QoS flows. Once the preconditions are met, it is now time to alert the subscriber. So, the called party device sends back the 180 Ringing message to say that the called party device is now ringing. So, at this point, the ringing tone will be heard by the calling party. When the called party answers the phone, a 200 OK message is sent to the original INVITE message by the called party. Now, the call is in place, and the last thing to do is just to acknowledge the called party with an ACK message. VoNR call procedures Calling party Called party INVITE