On our side, this happens when we have the phone point to it's SECONDARY ICP controller in DHCP. I believe the phone tries to contact it's primary controller after each call. In our particular case, I forced our phones on a remote campus to use the secondary controller, because for some reason, when contacting the primary controller, the phone gets stuck at the HTML Apps upgrade screen. So I had to install a firewall between the phones at that site, and the primary controller to block access. Then I made dhcp send those phones to the secondary controller. Phones come up now, but they spuratically reboot, usually at the end of a call.
check your IP Telephone Inventory (under maintenance and Diagnostics) and see where your phones are. Verify your Home Element for the phone (in user and device config) matches what you are passing for ICP in DHCP.
