Dial-up on Ubuntu ( or any Linux system for that matter ) has always
been problematic. First hurdle is the make of dial-up modem fitted as
some can be coaxed into working with Linux but some will not work no
matter what you do - all down to the way that manufacturers handed
over control to the OS and only considered Windows in their planning !

So the first thing to determine is the make and model number of your
dial-up modem - since you have Windows and obviously must use it to
get on-line currently then that would be the easiest way for you to
find this. Click on [Start] then Right Click on [Computer] and select
'Manage' from the drop down list. Then go to [Device Manager] and look
at the 'Modem' section to see what it says about the one in your PC
then report back.