Hi Roberto,
NetTalk requires the browser to support cookies, and JavaScript. (Not Java, just JavaScript).
If you get an error in script message, then that would point to something being wrong with the script.
If they're using a really bad browser there may also be a problem uncompressing compressed files. So you make have more success turning off the "pre-compressed files" option - although this results in a slow-down for everyone.
Cheers
Bruce