have you setup the Baud rate properly ?
--
To activate FWP LG LSP340E, press (Menu)->(2:Additional) ->(4:SIO Baud rate) and select 115, 200 bps.
To activate FWT LG LST250 DIAL' # # 7 # 0 * '
----
please post it here if you get it working with your laptop because i also have laptop with usb only but have never tried to connect with serial converter.