Sorry for taking a while to reply to this, I've flagged this up to the firmware team and they're investigating.
I've also checked back through the beta test forum for R36 and I can only find one specific report about someone having an issue with SIP and this user said that they managed to solve this issue by turning off uPNP in the Super Hub. If you're seeing this issue could you try that and report back on if it helps or not?
Not VOIP related. But since R36 I've been having exactly the same issue with my own (normal) router plugged into the SuperHub (in modem mode). Pre R36 it was fine, post R36 every 60minutes the router loses connection and needs restarting. For the time being I've taken it out of the chain and put the SuperHub back into router mode. Not quite the same issue, but similar symptoms.
For anyone not familiar with routers, it's worth mentioning that you will need to fix the IP address of your phone in the DHCP settings, otherwise its IP address will change on reboot and the forwarding will break.
Glad to hear that's worked out for you, I've marked that last answer as a 'Helpful Answer' so it should show up at the top of any search results on this topic. Will also make sure the firmware team and our front line agents are aware.