VoIP firewall settings and dscp marks

Hi.
For a past few months I am trying to connect my pbx system to my provider’s voip server.
Originaly i should use voip on siemens sx763 router that connects to voip server through layer3 forwarding and dscp. Right now I’m using rb750g as main router and dlink 584t as modem (wan).
Can anyone help me to setup firewall so that I can connect my pbx to voip server?