im pretty sure the answer to this but i want to make sure:
i want to connect pbx A to pbx B (two different brands) over IP. the cheapest I can come up with is to put a router with an fxs and fxo at each location and program the gateways as h.323; manually configured dial plans and peers. pretty much, i would program each pbx with a trunk access code, activating the fxs port, and send the digits to the other router, activating the fxo on that end, and acting as the bridge between the two pbx's. i need to know if any call control software needs to be installed also for the gateways to be operational.
again, i think a ucm is not needed, but i'll throw this out to you guys. tia
All you need are two routers. I just want to suggest you to use digital interfaces if possible, i.e. ISDN BRI since they are more reliable and have better audio quality.
If you don't have much experience with VoIP it would be better to contact a Cisco partner that will save you much of the troubles you'll probably have when you start configuring the system.