Unforetunately there we don't have enough detail from you to give you a detailed answer.
You will need to be a bit more specific about the signaling the PBX is using - E1 CAS (such as E1 R2) or ISDN PRI (Net5 or QSIG, network side or user side).
We also need to know if you are using a stand alone Gateway with or without a CallManager - in case you want to use MGCP, H323 or SIP for the VOIP signaling.
We would need details of the dial plan being used on the PBX and towards the VOIP network as well.
If you can get us these details we should be able to give you some samples to work with.
