aight ... i'm assuming you are using kppp
so start kppp>configure>device
see the top drop down box?
check your modem, as to what com port it's in (it's external right?)
if it's on com 1 set the value in the box to /dev/ttyS0, if it's in com2 /dev/ttyS1
If it's an internal modem ... ... good luck (never configured one of those under linux)