The mission: “save some bucks by using a free PBX using a cheap isdn card”. Don’t try! Buy something working in the first place. But if you have to, here is one example how it can be done. There are, for sure, many others!
The idea was to replace trixbox using an AVM Fritz!PCI card with something more up to date and not that buggy. FreePBX Distro kicked itself out because of the issues with mISDN. Elastix brought in mISDN support but still failed in configuring it. Since the setup was for only 3 users for now and the idea was to later buy something more professional (I really hope it comes to this point), I used Starface free. It has 4 users and 10 extensions for free. Yet the free version only allows using SIP providers. Also it was not possible to buy a Patton SmartNode 4120 at the moment, which I still hope to get somewhere in the future. So I needed to build our own SIP trunk since the provider used (M-NET) does not provide SIP trunks as a product.