I recommend that you turn the SIP ALG off in the router. It causes more headaches that it solves. It was really designed for computer based voip type software issues getting through NAT and firewalls. Skype, softphones, etc. But if you're using the grandstream ATA, then your router doesn't even know that it's VOIP/SIP. It's just another IP address. So ALG could actually negatively affect it.

Now, having said that, I never suggest someone put their Analog Telephone Adapter (The grandstream) into the DMZ. Except for troubleshooting. Truth be told, it's probably not a big issue. As long as it works, go for it. But for people who prefer to protect and secure their home network, you don't want anything in the DMZ unless it's intended to be. I have no problems giving the ATA a static IP (Which you've already done), giving that IP the highest priority in my QOS settings, and disabling the SIP ALG. If you want, the ports used in VoipO are on this forum, you can simply port forward. But if you're OK with the DMZ, then that's fine.

But definitely turn off the SIP ALG. Mike.