If the devices are in the same account they automatically get access to each other's services.
If devices are in separate accounts:
Put the remote Obi in the circle of trust of the US obi. This will give the remote Obi access to all services available on the US obi.
Without any further setup the remote obi will get the auto attendant when calling into the US obi, and calls can be completed using 2 stage calling. Two stage is where the caller chooses 2, to make a new call and then dials the same as from the phone connected to the US obi. (meaning dialing 7, 10 or 11 digits as required by the service. Calls dialed without a prefix will go out on the default line. prepending **1, **2 or **8 will route the call out of the chosen line. Speed dial also works from the remote obi.)
For direct dialing from the remote Obi:
The US obi should automatically appear in the speed dial list of the remote obi. (example speed dial location 4)
From the remote Obi dial 4 * 1 xxx xxx xxxx and the call will be placed to the number using the default outgoing line set for the US Obi. If the default service accepts 7 or 10 digit dialing you can use that instead of the 11 digit example.
Dialing 4* from the remote Obi sets the Obi in speed dial location 4, as the default gateway for the call. After pressing 4* dial the same as from the phone port connected to the US obi. (example: 4* **8 xxx xxx xxxx sends the call out on the line port of the US Obi.)