Hello kalshetty,
You will need at least an FXO port if you want to get a tone from the Siemens PBX into your IP network, that FXO will be your gateway out of the IP network (on which the router is connected). With the FXS port you will only be able to connect a tone from the router to the PBX or to a classic telephone, and in that case you won't be able to dial a number attached to the PBX from your IP network, only vice-versa.
Another way to do it it is through a E1/T1 port on the router (of course if the PBX has one too). This way you can have multiple tones from the PBX into your network, meaning that multiple users can call out through the PBX the same time.
The way to configure the router is described in detail on the Internet and depends on what kind of hardware you have.
If the Siemens Hipath 4000 v4 has IP capability you can connect them on Ethernet (fast, giga or whatever) interfaces (perhaps this will help with the configuration: http://www.cisco.com/c/en/us/solutions/collateral/enterprise/unified-meetingplace/1041242.pdf but it is for v 5)
Best of luck,
Narcis
