Assuming that re-invite is indeed the root cause of disconnection after 15 minutes, I would suggest change the settings on OBi instead of tinkering with the router. Take a look at the values in Service Providers -> ITSP Profile X SIP (where X is A or B depending on your setup). By default, RegistrationPeriod is 60 seconds, which should be more then enough to re-register in time. Also take a look if any of the "Expires" settings are not default. I've been using OBi with Voipo for about a year (first 100, and now 202), and never experienced this problem

Regarding suggestion 1 above - I wholeheartedly agree that it's a good idea to disable SIP ALG, but I can't see how it would help in this situation.

Suggestion 2... try it for troubleshooting to identify the root cause, but then put it back and fix the root cause. Leaving OBi open seems like taking unnecessary risk. Just sayin'