I think in almost every case, we end up using a direct connection to the MIPU. I have had some IT try to say that they can get it to work with NAT. I say to them, well let Toshiba engineers know of your miracle solution because so far this has never worked with the MIPU. In the case of the ZyXel, Toshiba actually took the time to make a step-by-step example of how to firewall the MIPU using this specific router. Years ago in Toshia's class, we tried to do the same thing with a SonicWall router, but we could not get this to work in the lab environment. I am sure that there is a solution with other router models. I no longer have the time, or desire to figure out Toshiba PBX solutions in my office lab.

Sonicwall does work well with SIP trunks to Toshiba behind a NAT using ALG (SIP transformations). I have several SIP trunks working through Sonicwall. The Toshiba IP phones do not use SIP. That is why you won't get them to work through NAT.