I have more OpenBSC running on different IPs and using different configurations, and I need to connect them together, so that it is possible to have, for example, two mobile phones connected to two different OpenBSCs and able to make calls with each other.

you can run openbsc with lcr at each location. two lcrs can be connected with E1 link. this link must not be physical, mISDN provides a l1oip driver (E1 or S/T over IP).

andreas