I'm just guessing, but slow and non-response might mean an IRQ conflict. Since you said it is on ttyS3, type "dmesg | less" to look through the kernel messages right at bootup. See if it picked up the port and assigned it the right IRQ. There is a Modem howto which has a lot of good information. It may be installed with Linux already, or check for it at http://www.linuxdoc.org. Lastly, were you using wvdialconf to try and detect the modem?
