The unit was working fine for years, rebooting once every 1 or 2 months. Then all of a sudden in the last 6 months the reboots got more and more frequent reaching the point were they happened once every 2 to 6 hours, even when in the middle of a call! I spent hours looking around the net and looking at syslogs with no real fix. I was waiting for the new firmware which hasn't materialized and I was about to chuck it in the bin and buy a new unit. In the middle of all this I got hold of a better router  and I noticed in the firewall logs that there was traffic on port 5060 from all over the world !  Then it hit me, that the unit was being hit by all sorts of suspect packetsc. My solution? Use a different port than 5060 or 5061. There are 65535 ports, use anything but the standard SIP ones. All that changes is that at the end of the SIP URI when someone needs to call you they add ':PORT' . That worked for me and it's over 20 days that I had a reboot. Give it a try and see if it works for you.