In any case, the optimal solution to your problem is to put the AirPort Extreme into "bridge mode" i.e. turn off the NAT, and use static, globally routed IP addresses. The next best solution is to figure out how to use STUN with your VoIP setup. I wouldn't expect SIP to work with the "Default Host" feature. I would expect a limited measure of success with the "Port Mapping" feature, but it would be a headache.