Hi Simon,
This is not expected behavior. Make sure your network switch is current on firmware. We have recently found a 3rd party switch was causing phones to lose registrations intermittently and cause the phones to reset making it appear like they are rebooting. System logs may also give some idea of what is happening (Status->Support Tools -> Logs).
We have also seen a bug where if the WAN IP address changes while on a call the system reset.
Chris
