Fun fact: H.323, the VoIP protocol, is pretty much just ISDN (specifically, its signalling protocol, Q.931) over IP. ISDN PRI runs over T1/E1, so I would imagi

Fun fact: H.323, the VoIP protocol, is pretty much just ISDN (specifically, its signalling protocol, Q.931) over IP. ISDN PRI runs over T1/E1, so I would imagine any T1/E1 PCIe card would do, if you can find the right software for it. Some Cisco routers, including old ones, accept modules with ISDN BRI ports, though I believe these are for connection to a service provider rather than a phone. Cisco IOS has VoIP functionality (if the right licences are enabled) and it can also speak ISDN over a Cisco E1/T1 module. There's also old PBX equipment you can find secondhand which speaks ISDN; for example, someone I know has a Cisco 2951 connected via T1/ISDN PRI to an old Meridian PBX, both purchased secondhand. It even has Cisco IP deskphones attached to the Cisco router which can call through to Meridian deskphones attached to the Meridian. I'm not personally familiar with any of this, though if you contact me I can put you in touch with some ISDN hobbyists who are. https://www.devever.net/~hl/contact https://www.devever.net/~hl/contact