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).
This would require setting up SIP support in each OBi and configuring Voice Gateways to initiate calls using SIP URI's, as the OBiTALK Service requires the use of an Obihai provided server.