can i suggest something... because i think its maybe
in the modem setup commands... you have 2 pc's why not
monitor with hyperterminal of windows, what ubuntu is
sending to the modem until it dails..simply the TD of
the linux serial port to the RD pin of the windows com
port and on the windows com port you also connect RTS
and CTS together....simply like a "null modem cable"
make sure the baudrates,start,stop bits and parity are
the same for both computers you have to see things as
"AAA"..."AT...." those ATxxx are commands for the
modem AT DTphone number... and so on