As it may cover the topic, I had problems with my old router and VoIP (United Internet), too. Problem was outgoing conections on random ports. But with pfSense and static port option in NAT, the problem disappeard automagically. The only thing you need is the one NAT rule mentioned and thats it. No Problems here anymore. Without any other software as e.g. the already mentioned proxies, you'll have to use a NAT rule.
Grey