Of course ISA modems exist.
At one time ISA was the only connection type there was, Video Cards, Network Cards, Modems, SCSI Adapaters, and everything.
And yes there are ISA Software Modems (WinModems), but they are not near as common as PCI SW Modems.
If memory serves the Creative 56k Flash II ISA is a hardware modem but I'm not 100% sure on that..I've been on broadband for over 2 years and don't really remember my old modem all that well.
But I do remember it was a very good and reliable modem it rarely let me down and most of the time it was my ISP's fault not it's.