toshiba tecra 730cdt: trouble installing

I have a 730cdt with internal toshiba modem (not pccard). When I try to install it in windows 98, first it’s required to install a special “toshiba modem port” for it, then select this port for modem. Win98 can’t find port an modem. When I install this port manually, It doesn’t appear in device manager, and have no assigned IRQ. So modem do not work (error opening port). Can anyone help with advice? The modem is called “Toshiba internal modem 28.8 (Voice+Fax+Data)” as it written in it’s .inf file.