Audit and tune shorewall QoS for branch office pbx

We have 2 offices with shorewall firewall as internet gateways running on centos5. There is a trixbox PBX at each site. The trixbox servers are configured to route intra-office calls over the internet to each other. We have implemented and tuned traffic shaping and QoS using shorewall tcclasses, tcdevices and tcrules running on centos5. We give intra-office VOIP the highest priority. Call quality is mostly good, but occasionally the calls break up. We can directly attribute the poor quality to the sending/receiving of large emails or other such internet traffic in spite of our attempts to make this type of traffic lower priority. I need an expert to audit the shorewall configuration and tune it to resolve the call quality issues. Also, the expert will need to explain the corrections that are made. Teach me how to do it correctly next time. Attached are the tc configuration files for one of the offices.