One thing you could try, if you know how, is to put your OBi in your router's DMZ (essentially allowing it unfettered access to the Internet). You can only do that with one device in your local network, but sometimes it helps.
Just out of curiosity, do you use DSL for your broadband connection, and if so, is there any chance your DSL modem has a built-in router? Many DSL routers do, and when you put another router behind the DSL modem it can cause "double NAT" issues for SIP connections. Putting your device in your router's DMZ might avoid that issue, although there may be security implications that I'm unaware of in doing that.