Internal PCI ISDN card.
Although I don't use ISDN anymore I used to with BT Speedway PCI cards. At the time these were reletively cheap and worked well under Windows and Linux. I used isdnutils which I believe aren't in vogue anymore so won't discuss that. The trick to getting the card to work was loading the hisax kernel module with the correct parameters
hisax type=27 protocol=2 id=line0