My particular modem was a modem riser for an MSI MS-6340 (aka K7TM Pro) motherboard.
Windows ME drivers reported the card as a HAMR56 or SmartRiser56 Voice Modem but Windows XP refused to recognise it.
After a lot of searching around I got this driver at www.smartlink.com - it seemed very similar to one of their products.
Just install the card - WinXP won't recognise it. Then run the file. It will self extract and install. WinXP will then detect the new devices and all the features will be available.