There have been posters that have gotten VOIP to work behind m0n0wall.
I beleive you will need to open up some UDP ports.
While I did get VOIP to work behind m0n0wall, the quality was not very
good. I opted to put the VOIP hardware in front of m0n0wall. My
particular VOIP vendor is Vonage. They shipped me a Linksys
Router/Firewall with VOIP enabled. My Lan looks like this.
In this configuration I didn't need to open any ports, as that is
taken care of by the Linksys router. Coincidentally, there is no
blocked packets logged in m0n0wall.