It is possible to find some ISDN -> SS7/C7 converters, but don't expect much in the way of debuggability or flexibility. I've used a few (Encore comes to mind) with limited results. the standard protocol for interconnection is probably EuroISDN, fo...
I think you're going to need to specify more about the requirements of your project. Try starting with the type of PBX, its interconnections and capacity requirements. You might also put some kind of an upper boundary on your budget since 'cost eff...