This depends upon what type of interface you have to the ISDN. I assume you have either Basic Rate Interface (BRI) or Primary Rate Interface (PRI). Do you have a DLL to control it? If so, you can use the "Call Library Function Node" VI to access this DLL and just make calls to it. If you have an ActiveX Server, you can use the LabVIEW ActiveX Automation VIs to access it.
J.R. Allen