Look at kernel booting messages, you can do
it after boot with
<shift>+<pgup> it will scroll up.
You can find that the kernel assigns irq4 to ttyS0 and irq3 to ttyS1.
The kernell does not listen to BIOS.
So when your modem gets to be recognised the irq is already busy.
The solution is to change the irq assignment of serial ports by setserial.
Hope this helps.
