Boy this is a super easy one on the OBI.
I thought the ph,ph2 setting was already set by default. I would call each of the numbers just to see if it doesn't already work how you want out of the box. If it doesn't then complete the following.
Go to whichever service provider you don't have a phone plugged in the following manner. Im assuming Service provide 2 below.
Go to Voice services>>SP2 Service
On that page go to X_InboundCallRoute
Change it to the following.
Then you are golden. I like to change the ring pattern using X_DefaultRing just so I can tell which number is being called. It's on the same page.
This setup only rings all attached phones to the obi for the account. There are more complex ways to do this if it doesn't quite do what you want. If so post which Obi you are using so we can explore the other options. You wouldn't believe what an OBI can do with a little knowledge.