Voip deployment

I am currently trying to design a voip network that works like this.
I have a cisco 2800 series which will act as a voice gateway and a PBX that connects analog phones. One port on the router goes to the Telco and the other to the PBX. My question is this , do i connect the PBX to an FXO port or an FXS port?

http://yfrog.com/b9diag1j

Plug the PBX to the FXS port.

Think of it like this: the O in FXO stands for office so remember that it points to the central office (the phone company). With that said, you can remember that the other card-type left (FXS) must point to the device that will receive the service (i.e.: the PBX)