The QOS your phone has an impact on the quality is the Transmit port. You can assign TOS, Diffserv and/or send it through VLAN dedicated to Media.
From Phone point of view you can configure nothing that can impact the QOS from the receiving port on the phone.
Nevertheless since the RTP is coming from the Asterix server you can configure the ASterix server Transmit port(Receive port on the phone) with a specific priority and/or trasmit through media specific VLAN.
Normally the recive ports and in some cases the transmit ports(or range) are not hard coded. At the entry point to the LAN you can still configure the ALG, etc at the entry point to assign QOS parameters that will help the RTP packets have a good QOS.
Whatever you do configure at the entry point of RTP packets to LAN have no effect on what is going on in the Internet/WAN.
Based on the RTCP measurements the SErver can disconnet or ask for a specifc QOS from the remote site.
The ALG must be able to detect the UDP packets as RTP to be able to do that in case the receive RTP ports are not hard coded.
