Hello.
I don't know if I will be of much help. I have some experience with Sonicwall, but not enough to configure a public IP to another port. I can, however, Google as well as anyone. In other routers I have heard the public IP address pass-through refereed to Proxy ARP. In Sonicall I've heard it called Transparent IP Mode (Splice L3 Subnet).
These 2 help pages seems to refernce this:
https://help.sonicwall.com/help/sw/eng/5710/25/9/0/content/Ch27_Network_Interfaces.031.24.html https://help.sonicwall.com/help/sw/eng/6920/26/2/4/content/PANEL_editInterface.htmlYou are correct regarding the MIPU. If you have any devices outside of your local network that are not connected vial VPN, then you will need to have the MIPU assigned with a Public IP address. You cannot NAT behind the firewall. The MIPU must have a public IP address assigned to it.
In at least 90% if my setups I will use another data switch as a DMZ to connect the MIPU to the Internet. I have been in several situations where I asked for a public IP address through the firewall, only to have the connection fails.
Toshiba says that they support and even recommend putting the MIPU behind a firewall, so I am assuming that if you can get the Sonicwall configured then this configuration should work. With the MIPU behind the firewall you would have the added benefit of being able to apply bandwidth management to the VoIP traffic.