I had the very same issue in a school environment. It was a network issue, but it wasn't set up with a VLAN. The phone system resided on its own subnet. In cases where the phone had to operate as a Layer 2 switch, a VLAN was created that the phone was tagged for, but not the rest of the system, and it works fine. Do you have VLAN tagging set up in MBU programming?
In the ICON programmer, under Communications, go to error logs and select all in the pop-up window. When it completes save the file somewhere you can easily find it. Go to Tools>IPKT reboot diagnostics, use the error log file you just downloaded for a list of all sets that have rebooted. You may get a clue as to why it's happening.
Also under Tools, if you're onsite, disconnect from the system and close your system data base. Click on SNMP diagnostics, check the LAN2 IP address and the window will populate with buttons for the ARP table, IPKT ports and other info showing connected IP addresses and more clues as to what is going on. The IPKT ports button will show every IP phone you have connected with IP and MAC. You have to select eacj IPKT for more fill-in info. It takes a few minutes to fully load , but it is well worth it. You can merge your system data base with this list to match extension/name with an IP and MAC, then export the file so you have a full listing of extsion,name, IP and MAC. Handy to have.
My similar issue was a network issue that the customer's IT department ultimately fixed. You may have to assign static IPs to all your phones to avoid using an IP address from the other VLAN. The IPKT port tables will show you how each phone is connected, the IP address it's using,whether it is static or DHCP and other configuration info that may help you out. The network guys are going to suggest firmware upgrades for the KSU, firmware upgrades for the phones and all manner of things not in their network, but the sticking point is absolutely a network issue.
Good luck with this, let us know how you make out.
