I have a 486/33 IBM clone with two serial ports (com1, com2) and mouse port.
Both the serial ports are directly sitting on the mother board.  I tried to
install a 2400 buad Hayes internal modem, but it doesnt work.  Once I dial any
number the system locks.  I do not hear any click or ring before it the system
hangs.  The modem has a 2 pin dip switch to select the appropriate port.  Once 
I change the settings on the dip switch, the system starts working again.

Then I bought a 2400 baud Hayes external modem and checked the system.  It
works fine with both the serial ports.  The internal modem has been checked on
another machine and it works fine, but does not work on my machine.

Can anyone please help me to find the problem.