I have not used such an ISP before, but Debian has a method of dealing with it if it is a proxy server your are connecting.
setup manual proxy and click "details", "use authentication" and supply username and password.
(smilies inserted somehow... username colon password at proxyipaddress colon port)
port is often 8080 or nothing for transparent proxying.
You will have to log out/in to pick up the changes to .bashrc or your can just run it:
To do this for all users, edit /etc/bash.bashrc or /etc/profile or /etc/skel/.bashrc to pick it up for new users.
From man bash:
When bash is invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. The --noprofile option may be used when the shell is started to inhibit this behavior.
When a login shell exits, bash reads and executes commands from the file ~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is started, bash reads and executes commands from /etc/bash.bashrc and ~/.bashrc, if these files exist. This may be inhibited by using the --norc option.
The --rcfile file option will force bash to read and execute commands from file instead of /etc/bash.bashrc and ~/.bashrc.