You first need to double-check that /dev/ttyS0 is actually the correct serial COM port you have the modem connected to.
Please look under System->Hardware and then click the 16550A-compatible COM port in the LH pane.
Then in the RH pane click on 'Advanced', and find the serial.device string or linux.device_file.
This will be something like: linux.device_file strlist /dev/ttySx where x == 0|1|2|3 serial.device strlist /dev/ttySx where x == 0|1|2|3
Please can you share with us what the reason was for the problem, so others with similar problems can be enlighted, including myself?
Well it's a bit ennoying. I've done all the given check list and got nearly no answer for a solution. So I decided to open the box to see that the serial device connector was partly unplug from the mother board; so it was the source of my problem.
I put it back in palce and it was done.
--- Michel Donais