an external modem doesnt need a driver for working. it just uses the serial port for data transfer. after plugging in the modem in serial port start the computer and after that start "kppp". this is used for configuring the point to point connection. after that add the new modem entry and select the serial port ...ie../dev/ttyS0...,if u have connected it in the 1st serial port.
then just connect to ur isp. the connection wud b just working fine....