I had an external one which meant it was fairly easy to setup as it just looks like a normal modem.
However, do remember that serial ports only allow 115kbps rather than the 128kbps that you expect on 2 ISDN lines. That's one thing to consider in choosing between external or pci. (I used to work for BT Speedway! blech! and indeed they *are* rebadged fritz cards) ;)
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