Hello,
since today most of new internet connections are bundled with Voice over IP this is a big thema not only for the future.
VoIP doesn't like NAT and so you need to do some tricky portforward with guessing of the port number on m0n0wall. With the right portforward the VoIP won't function 100% at all time, because of 24 hour disconnect of the internet provider etc. .
To solve this probleme you need to use the VoIP-box also as router und the m0n0wall get workless.
In professional situations, there are also more and more the need of route VoIP-traffic over NAT.
A simple solution for this problem is the integration of SIPROXD (
http://siproxd.sourceforge.net/) in m0n0wall.