If you have two or more OBi devices on the same LAN, served by a correctly functioning router, which is connected to the Internet, you won't need any special settings -- just dial the OBi number of the other device you wish to call.
In the above scenario, some routers require special configuration in the router and/or the OBi devices; a few are completely incompatible.
If you have two or more OBi devices on the same LAN, either not connected to the Internet, or they won't be using the Internet connection, i.e. you are trying to build an intercom-only system, you will have to configure each OBi with a unique static IP address and set up a configuration where each points to the other(s).
If you have only two OBi devices on a LAN, you can set them up as a hotline -- when you pick up one phone the other will ring.