Comment 3 for bug 32933

Your debconf priority probably isn't set correctly to be prompted for a proxy server.

The question to set the proxy server is at priority level medium. You can use "dpkg-reconfigure -pmedium msttcorefonts" and you should be prompted for a proxy server URL. A new version of msttcorefonts is in the archive (1.2ubuntu3) that will check for a proxy server set in the environment and if so use that to download the fonts.