If you have phones outside or off the internal network the MIPU can't be behind a firewall.
But from experience I can tell you it most likely a SIP attack on the IP cards. Which will cause them to shut down and or reboot.
That's great information, even if we have SIP turned off (which I believe we do), what is the solution then to stop this?
I believe that even if you are not using SIP, I believe that the MIPU will respond to SIP messages. If you even look at incoming data, once people see port 5060 open they start attacking it with SIP invites and registration messages, attempting toll fraud.
There is a way to put a firewall in front of the MIPU. The trick is that the MIPU must be assigned a public IP address without NAT. Toshiba had a document on how to configure a Zyxel router to filter incoming traffic while still giving the MIPU a public IP address. Then you could filter port all incoming traffic except for the ports the phones need for their IP phones.