I'm not sure if the Obi uses standard sip. If it does it should be able to accept a direct connection from a sip phone but I have tried direct inbound connections to my Obi with no luck. The local Obi Sip Uri should be obxxxxxxxxx@your.ip.address:10000
replace xxxxxxxxx with your obi number replace your.ip.address with your public ip address and 10000 can stay the same unless you changed the port manually.