PCI may work - mine didn't. The best to be on the safe side is a
serial modem... also better if you use more than one machine as you
can just push in the USB. Don't go wild on price... most serial modems
can be got as NOS on ebay for £1-£2 pounds. I got a BT enabler on
ebay.

You can have my PCI modem if you like... if you're in the UK.