What do you use to connect to the net? You use wvdial, Kppp or what?
If you use Kppp, click on configure, then click the modem tab. Select /dev/ttyS0 and see if it finds it by hitting connect. If not, try /dev/ttyS1 and trying that one. Sometimes /dev/modem will work if the link is there.
If you use wvdial, type in wvdialconf and go through that setup program. It should scan the serial ports for your modem and then create the config file for it. You may also need to configure ppp as well for you to stay connected. Just type in pppconfig and set up a new connection. For most ISPs, choose the default selection. I have to leave my DNS servers blank to. It gets those when it connects.
Either way, I see no reason it would need to edit fstab. That is a new one on me.
Hope one of those helps.