Marking VOIP packets with DSCP allows you to implement a QoS policy on your network. This will give SIP, RTP and IAX2 traffic a higher priority. Marking packets is not enough for QoS to work. Your network equipment (router, switch) needs to LOOK at the packets and honor the markings.
Here is a Linux iptables example which will mark VOIP traffic coming from your softphone, Asterisk, FreeSWITCH or any other VOIP software running at your computer.
# mark IAX2 packets iptables -t mangle -A OUTPUT -p udp -m udp --sport 4569 -j DSCP --set-dscp-class ef # mark SIP packets iptables -t mangle -A OUTPUT -p udp -m udp --sport 5060 -j DSCP --set-dscp-class cs3 # mark RTP packets, 10000-20000 port range iptables -t mangle -A OUTPUT -p udp -m udp --sport 10000:20000 -j DSCP --set-dscp-class ef
Packets passed through Voovox network are marked in the following way:
SIP signaling: set-dscp-class cs3 RTP audio: set-dscp-class EF IAX2: set-dscp-class EF
You may adjust your router setting to give such packets higher priority and bandwidth.