If he's running the voip server on the device itself which you can do with using optware and asterisk, then I think it would be opening up the ports, instead of forwarding them. Not sure if you could tell it to forward the port to itself.
I messed around with the asterisk server once. I think I even got it to connect using this firewall rule. Granted it only connected, and I wasn't ever able to make it send and received calls, but I lost interest pretty fast. This was with asterisk running on the router.
iptables -I INPUT -p udp —dport 5060 -j ACCEPT
If it is on another machine behind the router, then I would say to use port forwarding, I've been able to do it that way, and it's much easier to setup through the gui. Kinda fun to play with google voice using PBX in a flash that way. I got it to work but the quality was terrible, most likely because I had no QOS. I've got a different solution working now, but the QOS can be very important, and probably would have made my original setup work.
Is the server running on the router, or a machine behind the router?
edit: Just remembered, it may depend on the firmware you're running right now. I know it hasn't been until more recently that the "Sip" option under "Tracking / NAT Helpers" has been fixed, and then only on mods that have incorporated it. I'm not sure if that would cause an issue if that's enabled on a firmware where it hasn't been fixed.